Support `-c pylock.toml`

I'd be really keen on -c pylock.toml, and I think a lot of other users are too. Currently -c doesnt seem to engage the lockfile-aware parser.

-c pylock.toml would provide an excellent compatibility bridge for the bootstrap of future (more feature-complete) versions of pylock-aware tools, including future versions of pip itself.

Especially important considering that ensurepip vendors a (static; generally not up-to-date) version of pip, and getting it to safely upgrade itself to a known-good version is always a bit of a hassle.

Originally posted by @paveldikov in #13952