Change lru-disk-cache module to expose Meter and LruCache.
Change lru-disk-cache module to expose Meter and LruCache.

This module structure is preventing us from releasing with the `dist-client`
feature enabled. The release process removes "path" dependencies, meaning
we get the crates.io version of lru-disk-cache, which isn't exposing
lru_cache{Meter,LruCache} at any level.

mozilla/sccache,glandium/sccache,luser/sccache2,mozilla/sccache,mozilla/sccache,glandium/sccache,luser/sccache2,luser/sccache2,glandium/sccache,glandium/sccache,luser/sccache2