go/build: allow Import to ignore build constraints

[okdave]

There's no way at the moment to use Import or ImportDir to select all the files in package ignoring build constraints. There is the Package.IgnoredGoFiles field, but that does not extend to finding the dependencies of the ignored files, and so requires extra parsing steps.

One use case for this is in situations like App Engine: we want to select all the source files for an app, but not exclude source file that would be needed to compile a package in other/future versions of Go. So, we want to select files ignoring any go1.x tags.

/cc @dsymonds @adg

[minux]

why not just programmatically add all go1.x tags to Context.BuildTags (or even Context.ReleaseTags)? Ignoring all tags might not be what you want if you just want to select files for future Go versions.

[okdave]

Adding all go1.x tags isn't sufficient because then you will reject current versions: I don't want to select for just a specific future version, but rather all possible versions from now into the future.

AFAICT, the only way to achieve that with the current API would be to repeatedly invoke Import with increasing sets of tags: go1.0-go1.6; go1.0-go1.7, go1.0-go1.8, etc. Fortunately, you can use Package.IgnoredGoFiles to determine when that is necessary at all, and Package.AllTags to determine the set of version tags you need to try. But it's still very hacky.