Ignore build tags for packages in GOROOT too.

Current approach of using build.Import with UseAllFiles set to true may
result in MultiplePackageError and an incomplete package being
returned. Need to find a way to fix that. This is the best intermediate
solution for now.
Merge pull request #17 from shurcooL/feature/use-all-files-for-GOROOT-packages

Ignore build tags for packages in GOROOT too.