x/debug: change source file disambiguation heuristic.

First prefer files that match a longer suffix of the requested file.

Then break ties by choosing files that are under the directory from
which the program was compiled.

Then break ties by choosing a file with the shortest name.

Change-Id: I50b87fe4bf1fa70075e6d48032c6633c5c71f8da
Reviewed-on: https://go-review.googlesource.com/16260
Reviewed-by: Dave Day <djd@golang.org>