Add total result count to package list API

Refs #988

This commit connects the API "GET /package" endpoint to the persistence
layer `ListPackages()` method. The cursor request parameter is replaced
by the optional limit and offset parameters. The response now returns
the current page limit and offset, and the total number of results found
before paging, instead of a cursor value.

These changes will break the existing cursor-based pagination in the
Enduro Dashboard. I will fix the Dashboard paging in a subsequent
commit.

- Add optional `limit` and `offset` parameters to the GET /package
  request
- Add page `limit`, `offset`, and `total` fields to GET /package
  response
- Use `persistence.ListPackages()` to populate API results
- Add an adapter to convert goa `package_.ListPayload` search filters to
  `persistence.Filter` filters
- Add tests for the conversion of the package API parameters to
  persistence layer parameters, and the persistence search results to
  API response results

internal/api/design/package_.go

@@ -78,10 +78,12 @@ var _ = Service("package", func() {
 			Attribute("status", String, func() {
 				EnumPackageStatus()
 			})
-			Attribute("cursor", String, "Pagination cursor")
+			Attribute("limit", Int, "Limit number of results to return")
+			Attribute("offset", Int, "Offset from the beginning of the found set")
+
 			Token("token", String)
 		})
-		Result(PaginatedCollectionOf(StoredPackage))
+		Result(PackageList)
 		HTTP(func() {
 			GET("/")
 			Response(StatusOK)
@@ -92,7 +94,8 @@ var _ = Service("package", func() {
 				Param("latest_created_time")
 				Param("location_id")
 				Param("status")
-				Param("cursor")
+				Param("limit")
+				Param("offset")
 			})
 		})
 	})
@@ -319,6 +322,20 @@ var StoredPackage = ResultType("application/vnd.enduro.stored-package", func() {
 	Required("id", "status", "created_at")
 })
 
+var Page = ResultType("application/vnd.enduro.page", func() {
+	Description("Page represents a subset of search results.")
+	Attribute("limit", Int, "Maximum items per page")
+	Attribute("offset", Int, "Offset from first result to start of page")
+	Attribute("total", Int, "Total result count before paging")
+	Required("limit", "offset", "total")
+})
+
+var PackageList = ResultType("application/vnd.enduro.packages", func() {
+	Attribute("items", CollectionOf(StoredPackage))
+	Attribute("page", Page)
+	Required("items")
+})
+
 var PackageNotFound = Type("PackageNotFound", func() {
 	Description("Package not found.")
 	TypeName("PackageNotFound")

internal/api/design/pagination.go

@@ -1,6 +1,8 @@
 package design
 
-import "goa.design/goa/v3/dsl"
+import (
+	"goa.design/goa/v3/dsl"
+)
 
 func PaginatedCollectionOf(v interface{}, adsl ...func()) interface{} {
 	return func() {