sstatesig/find_siginfo: unify a disjointed API

sstatesig/find_siginfo: unify a disjointed API

find_siginfo() returns two different data structures depending
on whether its third argument (list of hashes to find) is empty or
not:
- a dict of timestamps keyed by path
- a dict of paths keyed by hash

This is not a good API design; it's much better to return
a dict of dicts that include both timestamp and path, keyed by
hash. Then the API consumer can decide how they want to use these
fields, particularly for additional diagnostics or informational
output.

I also took the opportunity to add a binary field that
tells if the match came from sstate or local stamps dir, which
will help prioritize local stamps when looking up most
recent task signatures.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>