pvscan: rework to improve PVs without metadata

pvscan: rework to improve PVs without metadata

Restructure the pvscan code, and add new temporary files
that list pvids in a VG, used for processing PVs that
have no metadata.

The new temp files, in /run/lvm/pvs_lookup/<vgname>, allow a
proper pvscan --cache to be done on PVs that have no metadata.
pvscan --cache <dev> is only supposed to read <dev>, but when
<dev> has no metadata, this had not been possible.  The
command had to fall back to scanning all devices to read all
VG metadata to get the list of all PVIDs needed to check for
a complete VG.  Now, the temp file can be used in place of
reading metadata from all PVs on the system.