layout.conf: make the pregenerated cache format controllable

layout.conf: make the pregenerated cache format controllable

Controllable via 'cache-format', currently it supports only one cache;
'pms', and defaults to it.  If an unsupported cache-format is specified,
the cache is disabled.  If pms is specified and metadata/cache directory
doesn't exist, the cache is disabled.

Finally, this rips out the best module support for locally overriding
the cache format used for pregenerated caches; this functionality made
zero sense (upstream determines the format, we use what is available).