package: normalize output of 'make external-deps'

For wget-downloaded packages, only the tarball name was displayed,
while for svn/git/bzr packages, the complete repository URL was
shown. As discussed on the list, what we want for 'make external-deps'
is only the tarball name even for git/svn/bzr downloads.

For local packages, we of course keep the full path since it's the
only information that we can provide.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>