go.tools/cmd/godex: better argument handling

1) Split a path.name argument at the last '.' that
   is not part of the path.

2) Try various importers always in the same order
   for consistent results (use lists instead of maps).

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/80790043
1 file changed