Resolver checksum calculation should be driven by layout

Currently, whether connector calculates checksums or not is driven by Layout.ChecksumLocation, but this is wrong, as some artifacts (like signatures) may not have checksum locations returned by layout, and hence, is shutting down complete checksum calculation. That, on the other hand, fails remote transport in case of FAIL checksum policy.
This change alone does NOT fix special cases like "resolving from remote signature", but makes a step to fix it.

The fact that Layout.ChecksumLocation is empty does not mean anything else, that resolver should not expect on remote checksum (REMOTE_EXTERNAL), but not shut down completely checksum validation as other mechanisms (REMOTE_INCLUDED or PROVIDED) should still kick in, if applicable.