go/importer: use correct path for path->package map

go/importer: use correct path for path->package map

In the presence of vendored packages, the path found in a package
declaration may not be the path at which the package imported from
srcDir was found. Use the correct package path.

Change-Id: I74496c3cdf82a5dbd6a5bd189bb3cd0ca103fd52
Reviewed-on: https://go-review.googlesource.com/18460
Reviewed-by: Alan Donovan <adonovan@google.com>