Go: cmd/go: do not update go.mod and go.sum with imports found in .gitignore

[mkungla]

go.mod and go.sum should not be updated from paths found in .gitignore which may contain resources or experimental code.

[ianlancetaylor]

I don't think the go tool should be aware of the Git specific .gitignore file.

See also #30058.

[ianlancetaylor]

CC @bcmills @jayconrod

[mkungla]

@seankhliao Can you elaborate your reaction?

[seankhliao]

Same as above, the go tool should not be aware of git ignore things and the dependencies for your experiments still need to be tracked somewhere

[mkungla]

Well, this behavior can now be achieved by placing an empty go.mod file in that directory.

[bcmills]

I agree with @ianlancetaylor and @seankhliao: the go command should not try to interpret vcs-specific files. Plus, we already have at least two mechanisms to ignore files: // +build ignore to ignore files, and dropping in a go.mod file to ignore directory subtrees. I don't see a pressing need for yet another.