bitbake/fetch: Spell out which fetcher backends support and recommend checksums

There were some hardcoded behaviours in the system for which backends
support checksums verses which backends recommend them verses which
don't recommend them.

This moves the functionality into specific fetchers and then makes the
general code generic. This cleans up the codebase and fixes some corner
cases such as trying to checksum directories returned by the git fetcher.

(Bitbake rev: ef6d268f7b8527541a7fb044cf95a973be4097f4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/git.py

@@ -82,6 +82,9 @@ class Git(FetchMethod):
         """
         return ud.type in ['git']
 
+    def supports_checksum(self, urldata):
+        return False
+
     def urldata_init(self, ud, d):
         """
         init git specific variable within url data