Allow use of a BUILD file with a suffix in a directory with no BUILD file without a suffix.
Also allow more characters in a BUILD file suffix.

Testing Done:
Manually renamed BUILD to BUILD.gen in src/java/com/pants/examples/hello/main

Updated test_build_file.py and test_linkify.py
Travis-ci went green at https://travis-ci.org/pantsbuild/pants/builds/25567951

Bugs closed: 126

Reviewed at https://rbcommons.com/s/twitter/r/381/