lib/package_remove.c: add a bit of debugging if pkg wasn't found in pkgdb.
util.c: add stricter checks for pkgver conformance.

Make xbps_pkg_{name,version} return NULL if next character
after the last '-' character is not a digit, and if there's a digit
make sure that there exists a '_' character too.

Added more tests to the testsuite to catch this. With these changes
'python-e_dbus' is properly detected and validated.