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.

src/pip/_internal/commands/cache.py

@@ -105,9 +105,9 @@ def get_cache_info(self, options: Values, args: List[Any]) -> None:
                     Package index page cache location: {http_cache_location}
                     Package index page cache size: {http_cache_size}
                     Number of HTTP files: {num_http_files}
-                    Wheels location: {wheels_cache_location}
-                    Wheels size: {wheels_cache_size}
-                    Number of wheels: {package_count}
+                    Built wheels location: {wheels_cache_location}
+                    Built wheels size: {wheels_cache_size}
+                    Number of built wheels: {package_count}
                 """
             )
             .format(
@@ -140,7 +140,7 @@ def list_cache_items(self, options: Values, args: List[Any]) -> None:
 
     def format_for_human(self, files: List[str]) -> None:
         if not files:
-            logger.info("Nothing cached.")
+            logger.info("No locally built wheels cached.")
             return
 
         results = []