cmd/go, flag: make undefined flag msg less ambiguous. Fixes #36364

[brianonn]

This fix tries to improve the error message from an undefined flag as discussed in issue #36364 Changing this error message unfortunately has a high impact on 3rd party tools and editors that monitor the go command stderr output for the error message that is changed here. Any tool that imports the flag package will be affected. Vscode-go plugin and likely other editors and tools will be affected. I have also completed the matching changes neeeded for golang/tools, golang/dep, golang/gofrontend and microsoft/vscode-go. I'll link them in here after I create the corresponding PR's for them all

golang/gofrontend#2
golang/dep#2238
golang/tools#216
microsoft/vscode-go#3113

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Message from Brian Onn:

Patch Set 2:

These are the related PR's that are affected by this change.

golang/gofrontend#2
golang/dep#2238
golang/tools#216
microsoft/vscode-go#3113

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Patch Set 2:

(1 comment)

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Message from Rob Pike:

Patch Set 2:

By the way, I'm not sure it's worth the disruption to the ecosystem. I'll let others weigh in there.

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Message from Brian Onn:

Patch Set 2:

(1 comment)

Patch Set 2:

By the way, I'm not sure it's worth the disruption to the ecosystem. I'll let others weigh in there.

Although I authored this change, I also agree with your sentiment here.

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Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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Patch Set 1:

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Next steps:
Within the next week or so, a maintainer will review your change and provide
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Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
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During May-July and Nov-Jan the Go project is in a code freeze, during which
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