Cache check_specfile_features() results
This is to allow 3rd party users call specfile_uses_rpmautospec()
multiple times about different sets of used features without having to
parse the spec file every time. It assumes that the spec file doesn't
change over the runtime of the process, if this can't be guaranteed,
call check_specfile_features.__wrapped__() directly or disable caching
when calling process_distgit() (like in the Koji builder plugin).
Signed-off-by: Nils Philippsen <nils@redhat.com>