Resolve symlinks in GOPATH or source file path in order to find correct package.
Fixesgolang/go#17515
Change-Id: Iaf7e85578fce040b329427ce6f51948a69e57a39
Reviewed-on: https://go-review.googlesource.com/33858
Reviewed-by: Alan Donovan <adonovan@google.com>