Clarify pip cache output

[vanschelven]

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.

Fixes #11300

[pfmoore]

You'll need to fix the tests, but in principle this seems reasonable to me.

[sbidoul]

You could even maybe use "locally built wheels" everywhere.

[pradyunsg]

Assuming CI is green, LGTM.

[vanschelven]

You could even maybe use "locally built wheels" everywhere.

I was already doubting between "locally built" and "built" with the former being more explicit and clear and the latter being more brief. On second thought, brevity isn't really a consideration here if you look at the surrounding lines, i.e. "Package index page cache location:" is still longer than the longest line with "locally built wheels" would be, and given that we have a +1 on "locally built" I'll adapt accordingly.