Add more detail on platform mismatch errors

[sepehr-rs]

Fixes #5003

When pip can't find a matching distribution, it now shows why each available distribution was rejected. This replaces the old unhelpful message "No matching distribution found for X" with a summary of skipped candidates, grouped by rejection reason.

Example:

(pip) sepehr@debian:~/w/pip$ pip install numpy==1.19.0 --target linux_aarch64 --only-binary=:all:
ERROR: Ignored the following yanked versions: 2.4.0
ERROR: Could not find a version that satisfies the requirement numpy==1.19.0 (from versions: 2.1.0, 2.1.1, 2.1.2, 2.1.3, 2.2.0, 2.2.1, 2.2.2, 2.2.3, 2.2.4, 2.2.5, 2.2.6, 2.3.0, 2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.3.5, 2.4.0rc1, 2.4.1, 2.4.2, 2.4.3, 2.4.4)
Skipped 3802 unsuitable distribution(s)
  183: unsupported format
    - numpy-1.0.1.dev3460.win32-py2.4.exe
    - numpy-1.3.0.tar.gz
    - numpy-1.3.0.win32-py2.5.exe
    ... and 180 more
  3619: incompatible platform/tags
    - numpy-1.5.1-cp27-none-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.whl
    - numpy-1.6.0-cp26-cp26m-manylinux1_x86_64.whl
    - numpy-1.6.0-cp26-cp26mu-manylinux1_x86_64.whl
    ... and 3616 more
ERROR: No matching distribution found for numpy==1.19.0

One limitation is that printing every rejected candidate makes the output grow very quickly (over 3,000 lines in the example above), and this caused several tests to fail. Because of that, verbose mode currently shows the same summarized output instead of listing every rejected candidate individually.

I’d welcome any feedback on whether this trade‑off seems reasonable, and whether it would be useful to expose the full list of rejected candidates in some other way (for example behind a separate flag or with additional safeguards).