extract mdx (libraries) field as test-scope use, declare matching deps

monopam lint: the [(mdx ...)] stanza's own [(libraries ...)] field now
surfaces as test-scope library use, alongside the existing detection of
the [mdx] stanza itself. Without this, libraries that exist solely to
satisfy README mdx blocks were flagged as unused.

Then declare the freshly-required [:with-test] dependencies across the
packages whose mdx blocks pull in extra libraries (eio_main, fmt,
requests, nox-tls, nox-csv, nox-git, nox-http, cstruct, etc.).