refactor: Simplify exported API

There are too many exported identifiers in package godfish. A developer
interested in making a new driver implementation really just needs to
know about the Driver interface and not much else. Split up the code so
that the only exposed functionality may build:

- a Driver implementation (pkg godfish/drivers/...)
- a standalone binary (pkg godfish/internal/cmd)

The material that was in pkg godfish is important to get that working,
but is probably not necessary for outside consumers. Most of these
changes involve moving material from pkg godfish to pkg internal,
adjusting callers, and ensuring that pkg godfish's exported API doesn't
reference pkg internal.

Other notable changes are:

- Change UpdateSchemaMigrations signature. Putting the Direction type in
  a method signature complicates things IMO; in practice, it means up or
  down, so a boolean makes more sense here. The Direction type is useful
  for disambiguating labels for the direction in a migration filename,
  but that's not a concern for a Driver implementation.
- Remove func CreateSchemaMigrations because it's not used anywhere and
  the functionality is already in a Driver method.
- Move buildtime versioning info variables to pkg cmd because the
  Version type is now in pkg internal.
- Move validation for direction label to ensure it's always validated,
  not just in the subcommand.
- Move pkg info stuff into pkg internal, this simplifies dependencies.

Makefile

@@ -7,11 +7,11 @@ PKG_IMPORT_PATH=github.com/rafaelespinoza/godfish
 
 # inject this metadata when building a binary.
 define LDFLAGS
--X $(PKG_IMPORT_PATH)/internal/version.BranchName=$(shell git rev-parse --abbrev-ref HEAD) \
--X $(PKG_IMPORT_PATH)/internal/version.BuildTime=$(shell date --rfc-3339=seconds --utc | tr ' ' 'T') \
--X $(PKG_IMPORT_PATH)/internal/version.CommitHash=$(shell git rev-parse --short=7 HEAD) \
--X $(PKG_IMPORT_PATH)/internal/version.GoVersion=$(shell $(GO) version | awk '{ print $$3 }') \
--X $(PKG_IMPORT_PATH)/internal/version.Tag=$(shell git describe --tag)
+-X $(PKG_IMPORT_PATH)/internal/cmd.versionBranchName=$(shell git rev-parse --abbrev-ref HEAD) \
+-X $(PKG_IMPORT_PATH)/internal/cmd.versionBuildTime=$(shell date --rfc-3339=seconds --utc | tr ' ' 'T') \
+-X $(PKG_IMPORT_PATH)/internal/cmd.versionCommitHash=$(shell git rev-parse --short=7 HEAD) \
+-X $(PKG_IMPORT_PATH)/internal/cmd.versionGoVersion=$(shell $(GO) version | awk '{ print $$3 }') \
+-X $(PKG_IMPORT_PATH)/internal/cmd.versionTag=$(shell git describe --tag)
 endef
 
 test:
@@ -23,31 +23,31 @@ clean:
 cassandra:
 	$(GO) build -o $(BIN) -v \
 		-ldflags "$(LDFLAGS) \
-		-X $(PKG_IMPORT_PATH)/internal/version.Driver=cassandra" \
+		-X $(PKG_IMPORT_PATH)/internal/cmd.versionDriver=cassandra" \
 		./drivers/cassandra/godfish
 cassandra-test:
 	$(GO) test $(ARGS) ./drivers/cassandra
 
 postgres:
 	$(GO) build -o $(BIN) -v \
 		-ldflags "$(LDFLAGS) \
-		-X $(PKG_IMPORT_PATH)/internal/version.Driver=postgres" \
+		-X $(PKG_IMPORT_PATH)/internal/cmd.versionDriver=postgres" \
 		./drivers/postgres/godfish
 postgres-test:
 	$(GO) test $(ARGS) ./drivers/postgres
 
 mysql:
 	$(GO) build -o $(BIN) -v \
 		-ldflags "$(LDFLAGS) \
-		-X $(PKG_IMPORT_PATH)/internal/version.Driver=mysql" \
+		-X $(PKG_IMPORT_PATH)/internal/cmd.versionDriver=mysql" \
 		./drivers/mysql/godfish
 mysql-test:
 	$(GO) test $(ARGS) ./drivers/mysql
 
 sqlite3:
 	CGO_ENABLED=1 $(GO) build -o $(BIN) -v \
 		-ldflags "$(LDFLAGS) \
-		-X $(PKG_IMPORT_PATH)/internal/version.Driver=sqlite3" \
+		-X $(PKG_IMPORT_PATH)/internal/cmd.versionDriver=sqlite3" \
 		./drivers/sqlite3/godfish
 sqlite3-test:
 	$(GO) test $(ARGS) ./drivers/sqlite3

driver.go

@@ -26,7 +26,7 @@ type Driver interface {
 	// UpdateSchemaMigrations records a timestamped version of a migration that
 	// has been successfully applied by adding a new row to the schema
 	// migrations table.
-	UpdateSchemaMigrations(dir Direction, version string) error
+	UpdateSchemaMigrations(forward bool, version string) error
 }
 
 // AppliedVersions represents an iterative list of migrations that have been run

drivers/cassandra/cassandra.go

@@ -94,9 +94,9 @@ func (d *driver) AppliedVersions() (out godfish.AppliedVersions, err error) {
 	return
 }
 
-func (d *driver) UpdateSchemaMigrations(dir godfish.Direction, version string) (err error) {
+func (d *driver) UpdateSchemaMigrations(forward bool, version string) (err error) {
 	conn := d.connection
-	if dir == godfish.DirForward {
+	if forward {
 		err = conn.Query(`
 			INSERT INTO schema_migrations (migration_id)
 			VALUES (?)`,

drivers/mysql/mysql.go

@@ -92,9 +92,9 @@ func (d *driver) AppliedVersions() (out godfish.AppliedVersions, err error) {
 	return
 }
 
-func (d *driver) UpdateSchemaMigrations(dir godfish.Direction, version string) (err error) {
+func (d *driver) UpdateSchemaMigrations(forward bool, version string) (err error) {
 	conn := d.connection
-	if dir == godfish.DirForward {
+	if forward {
 		_, err = conn.Exec(`
 			INSERT INTO schema_migrations (migration_id)
 			VALUES (?)`,

drivers/postgres/postgres.go

@@ -65,9 +65,9 @@ func (d *driver) AppliedVersions() (out godfish.AppliedVersions, err error) {
 	return
 }
 
-func (d *driver) UpdateSchemaMigrations(dir godfish.Direction, version string) (err error) {
+func (d *driver) UpdateSchemaMigrations(forward bool, version string) (err error) {
 	conn := d.connection
-	if dir == godfish.DirForward {
+	if forward {
 		_, err = conn.Exec(`
 			INSERT INTO schema_migrations (migration_id)
 			VALUES ($1)

drivers/sqlite3/sqlite3.go

@@ -67,9 +67,9 @@ func (d *driver) AppliedVersions() (out godfish.AppliedVersions, err error) {
 	return
 }
 
-func (d *driver) UpdateSchemaMigrations(dir godfish.Direction, version string) (err error) {
+func (d *driver) UpdateSchemaMigrations(forward bool, version string) (err error) {
 	conn := d.connection
-	if dir == godfish.DirForward {
+	if forward {
 		_, err = conn.Exec(`
 			INSERT INTO schema_migrations (migration_id)
 			VALUES ($1)`,