odb: stop including "odb/source.h"

odb: stop including "odb/source.h"

The "odb.h" header currently includes the "odb/source.h" file. This is
somewhat roundabout though: most callers shouldn't have to care about
the `struct odb_source`, but should rather use the ODB-level functions.
Furthermore, it means that a couple of definitions have to live on the
source level even though they should be part of the generic interface.

Reverse the relation between "odb/source.h" and "odb.h" and move the
enums and typedefs that relate to the generic interfaces back into
"odb.h". Add the necessary includes to all files that rely on the
transitive include.

Suggested-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>