Remove some global state from vendor and recovery snapshots

Snapshots storead global sets of modules that should be replaced with
vendor snapshot modules.  Move the data instead to a vendor_snapshot
or recovery_snapshot module type that depends on all the modules in
the snapshot, and then have modules that should use the snaphsot
depend on it to query for the set of modules that should be replaced.

Bug: 177098205
Test: vendor_snapshot_test.go
Change-Id: I2826adacfb473e9139b5ea93ba83b8a54cc1a56b