Clarify pip cache output
Clarify pip cache output

pip cache reported 'Nothing cached' and 'Number of wheels: 0' even when things were
obviously cached. The reason, as per @uranusjr's comment is

1. If a wheel is downloaded (from a secure source), pip uses the network layer
   cache (provided by a requests extension implemented with cachecontrol) in
   ${CACHE_DIR}/http.
2. If a wheel is built from source, pip caches it separately in ${CACHE_DIR}/wheels.
3. The pip cache list command is currently only capable of showing entries in
   the wheel cache, not the network cache.

The solution is to clarify what `pip cache` is reporting about.

pfmoore/pip,pradyunsg/pip,sbidoul/pip,sbidoul/pip,pypa/pip,pypa/pip,pradyunsg/pip,pfmoore/pip