is_xml{,_like} handle paths, not filenames

The way is_xml and is_xml_like handle the pom.xml case, one seems to
expect a filename while the other expects a path.

This patch makes both functions more robust so they can handle both
path and filename, but won't incorrectly filter filenames that only
happen to end on "pom.xml" (say, fun-with-a-pompom.xml).

Change-Id: Id2653152b2e46bfa9b892a8f656cccaa44a8185a
Merge "is_xml{,_like} handle paths, not filenames"