Use Go Modules, Upgrade Module to Go 1.20

[adrianosela]

Note: I don't expect this large PR to be merged, would be great if an AWS internal would go over the steps outlined in this PR to modernize and modularize the repo.

Issue #, if available:

• Upgrade/Re-Vendor (or switch) UUID Library #73
• Reviving stale Modularise the repository #33

Description of changes:

• Initializes go module with go 1.20
• Upgrade Dockerfile to build with Go 1.20
• Manually set library github.com/twinj/uuid to the last compatible version (v0.0.0-20151029044442-89173bcdda19)
• Fix makefile to use go build instead of go build -i (flag does not exist in go 1.20)
• Run go fmt -s -w to go format the whole repo
• Delete vendor dir (rm -rf vendor) and Run go mod vendor to vendor with go modules

Steps:

On a system with the latest version of go (1.20 at the time of opening this PR)

• Remove the existing vendor directory with rm -rf /vendor
• (optional) Run go fmt -s -w . to go format the code base
• Initialize a new go module for this repo with go mod init
• Initialize go.sum file with go mod tidy
• Re-vendor deps with go mod vendor

Note that if you try to build now (with make build) go vet will complain about inconsistent vendoring

• Run go mod vendor again and try to build again with make build (this will fail, continue reading...)

Note that three steps will vendor a dependency with an incompatible latest-version (github.com/twinj/uuid v1.0.0), so building will fail with

# github.com/aws/session-manager-plugin/src/message
src/message/messageparser.go:170:10: cannot use nil as uuid.UUID value in return statement
src/message/messageparser.go:176:10: cannot use nil as uuid.UUID value in return statement
src/message/messageparser.go:182:10: cannot use nil as uuid.UUID value in return statement
src/message/messageparser.go:188:10: cannot use nil as uuid.UUID value in return statement
src/message/messageparser.go:194:10: cannot use nil as uuid.UUID value in return statement
src/message/messageparser.go:417:14: invalid operation: input == nil (mismatched types uuid.UUID and untyped nil)
src/message/messageparser.go:497:25: undefined: uuid.CleanHyphen
# github.com/aws/session-manager-plugin/src/message
vet: src/message/messageparser.go:170:10: cannot use nil as uuid.UUID value in return statement
make: *** [checkstyle] Error 1

• Open go.mod and change the version of github.com/twinj/uuid from v1.0.0 to v0.0.0-20151029044442-89173bcdda19
• Run go mod tidy and go mod vendor again. Try building again (with make build)... You will now get failures related to running go build -i (with an -i flag, which is not supported in go 1.20)
• Open the makefile and remove the -i from alias in the second line e.g.

-GO_BUILD := go build -i
+GO_BUILD := go build

• (optional) also remove some directories from the cleanup makefile target, e.g.

 clean:: remove-prepacked-folder
-       rm -rf build/* bin/ pkg/ vendor/bin/ vendor/pkg/ .cover/
+       rm -rf build/* bin/ .cover/
        find . -type f -name '*.log' -delete

✅ Building (with make build) should now succeed for all the OSes

Hopefully this gets done one day!

Testing:

None done. I imagine the SSM team in charge of the plugin has an end-to-end user acceptance test (UAT) and some magic to vet all still works well with the latest-and-greatest version of all dependencies.

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