proposal: x/tools/goimports: Look for .goimportsignore in modules

[smoyer64]

As proposed in #32780, we have a recurring need to ban or blacklist certain packages within a Go module. It was pointed out in that issue that we really should be banning packages rather than try to reject certain packages as part of the dependency graph.

Since these packages most often end up in our go.mod files via the automatic organization of imports by our IDEs and editors (using gopls and/or goimports), it seems that goimports is the proper place to effect this "exclusion".

Therefore, I'm proposing that goimports looks for a .goimportsignore file local to the go.mod file if executed in a directory hosting a Go module. I realize that packages can be globally ignored via $GOPATH/.goimportsignore but there is a definite need to control this behavior on a module-by-module basis.

[rsc]

goimports is not the only tool that consumes modules. We should solve #30058 instead of extending the .goimportsignore hack further.

[rsc]

/cc @ianthehat FYI.
Duplicate of #30058?

[rsc]

No response from @ianthehat, but I am fairly sure this is a duplicate of #30058. Closing in favor of that one.