Add MultiMod releaser app (source code and half of unit tests)

[eddyleelin]

This PR adds code to generate a Go Cobra application used for flexible module releasing for Go repos with multiple Go Modules. Currently, the script works as intended in testing with the OpenTelemetry-Go repo, and is intended primarily for maintainers of repos. For more context about this tool, such as why it is necessary, how to use it, and what the overall design is, please see this design doc.

The following are completed and included in the PR:

• support app subcommands
  • verify
  • prerelease
  • tag
• update README with usage
• unit tests have been added for all functions in common and verify

Here is the organization of the major source files changes (not including test-related or CI-related files):

- multimod/
    - README.md # README for the multimod tool
    - main.go # creates the Cobra app binary
    - cmd/ # handles CLI commands, subcommands, and flags using Cobra
        - root.go # define global flags
        - verify.go
        - prerelease.go
        - tag.go
    - internal/
        - common/ # shared functions for multimod app
            - commontest/ # package containing shared functions used for testing
            - conversions.go # convert between Mod import path, file path, tag names
            - tools.go # miscellaneous shared funcs
            - versions.go # structs/funcs for reading in version files
        - verify/
            - verify.go
        - prerelease/
            - prerelease.go
        - tag/
            - tag.go

There are several TODO items to clean up the code which will be implemented shortly in subsequent PRs, including a full testing suite to help maintain the tool. In addition, several refactoring items and additional features are currently in the works, such as:

• prevent script from deleting tags already in remote upstream
• use Go-Git for Git functions rather than CLI (in progress)
• enhance error messaging in verify to support slices of errors and update syntax
• add ability to specify multiple module sets
• add verification to tag script to check that the module set specified is the same as the one updated in the prerelease commit
• add functionality to run "go mod tidy" for all Modules in prerelease step

[codecov]

Codecov Report

Merging #8 (6007c84) into main (520d53f) will increase coverage by 9.43%.
The diff coverage is 39.69%.

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   30.00%   39.43%   +9.43%     
==========================================
  Files           1       14      +13     
  Lines          20      743     +723     
==========================================
+ Hits            6      293     +287     
- Misses         11      423     +412     
- Partials        3       27      +24     

Impacted Files	Coverage Δ
multimod/internal/prerelease/prerelease.go	0.00% <0.00%> (ø)
multimod/internal/tag/tag.go	0.00% <0.00%> (ø)
multimod/main.go	0.00% <0.00%> (ø)
multimod/cmd/verify.go	26.31% <26.31%> (ø)
multimod/internal/verify/errors.go	30.76% <30.76%> (ø)
multimod/internal/common/commontest/commontest.go	37.25% <37.25%> (ø)
multimod/cmd/tag.go	40.00% <40.00%> (ø)
multimod/internal/common/tools.go	53.84% <53.84%> (ø)
multimod/internal/common/versions.go	53.84% <53.84%> (ø)
multimod/cmd/root.go	63.63% <63.63%> (ø)
... and 16 more

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[pellared]

Disclaimer: Opinionated comment

I think it might be worth considering using https://github.com/goyek/goyek for making build pipelines in Go.
As an example I am trying to make some reusable tasks/pipelines here: https://github.com/signalfx/go-pipeline and here is a sample PR for adoption: signalfx/splunk-otel-go#71

Why? I think it would be easier to maintain, debug, and use than a combination of CLI Go programs + Makefile.

Tomorrow, I will try to create an issue about it under https://github.com/open-telemetry/opentelemetry-go regarding it. Then I will remove or edit this comment.

EDIT

Here is the issue: open-telemetry/opentelemetry-go#2152