go/buildutil: handle symlinks in filenames

Resolve symlinks in source directories (GOPATH, GOROOT, etc.) and source
files in order to find correct package.

Fixes golang/go#16219

Change-Id: I6fae14908827d5ebac55dfe455eaf616f71f8767
Reviewed-on: https://go-review.googlesource.com/33919
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed