cmd/go: add -modversion flag setting main module version on debug.ReadBuildInfo

[Fryuni]

As discussed in #29228 currently it is required very hacky workarounds to get the version of the built binary.
From my understanding the most used are variations of 'go build -ldflags="-X main.version=${version}"' and reading from the injected variable.

Currently building a binary using "go build" has a hardcoded "(devel)" version for the main module on the data for ReadBuildInfo. The program has to be installed from "go get" or "go install" in order for the version to be added.

This PR adds a "-modversion" to set the value for the main module version.

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This PR (HEAD: 1ba020b) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/281912 to see it.

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Message from Daniel Martí:

Patch Set 2:

(1 comment)

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Patch Set 2: Code-Review-1

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Message from Luiz Ferraz:

Patch Set 2:

(2 comments)

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Message from Daniel Martí:

Patch Set 2: Code-Review-2

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Message from Luiz Augusto Ferraz:

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