Prune files based on known unused build constraints

[lindner]

It would be valuable if a project could specify that they are not using a specific build extension, which would allow for pruning more dependencies.

For example in Perkeep we import go-oauth

  vendor/golang.org/x/oauth2/google/appengine_hook.go:
  // +build appengine appenginevm".

Which pulls in the full appengine dep

We would like to specify that we are not compatible with appenginevm and omit files and dependencies for those.

[sdboyer]

This actually gets really tricky to do, unfortunately - see #291.

i agree there's value to doing it, but it still seems like it's in the "nice to have" category - especially given that you can use ignored to achieve much the same effect.

(closing as dup of #291, which would necessarily cover this)