Move avocado.external.gdbmi_parser and avocado.external.spark
to avocado.core.gdmi_parser and avocado.core.spark, in conformance to
our purpose of keeping things not related to test developers,
inside avocado.core, isolated from users.

After this, `avocado.external` module is gone.
Signed-off-by: NRudá Moura <rmoura@redhat.com>