Allow '_' in checkstyle PackageName rule

Allow '_' in checkstyle PackageName rule

The JLS allows '_' in package names but the default checkstyle regex
pattern for the PackageName rule doesn't. Underscores shouldn't normally
be used but the yang code generator can generate package names with
underscores. We can exclude most yang-generated code from checkstyle but
the CSS code generator places some classes under src/main/java to be
completed (ie *Module classes) so we need the regex pattern to allow
underscores.

Change-Id: I14fdb1955499d5684b1bdaeabd188f7d425caf60
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>