maven_depmap: handle extensions with multiple dots correctly

When artifact specification was used instead of pom.xml, the extension of
artifact file should be verified based on classifier specified (jar by default)
and compared to filename of artifact given. We should not assume following:

    * Artifact has ".jar" extension (just use it as default_)
    * Assume extension contains single dot

This allows arbitrary extensions as long as they match information in depmap and
filename on local filesystem
TODO: remove add_maven_depmap fix (fixed in ba91eb89)