This consolidates all queries to the wrapdb to go through the open_wrapdburl()
function. The function handles a domain-specific url scheme
wrapdb. When encountered, it substitutes the scheme with https and
an authority(netloc) with either upstream wrapdb address or
a user-defined one from the MESON_WRAPDB_MIRROR environment variable.

Environment variables are prone to shell injection attacks, especially
on build servers that many people may have access to. Changing address in
a file requires an explicit command that is supposedly under the control
of a trusted developer.

This changes moves responsibility of setting wrapdb source to the
`wrap update-db` command. Improve wording.

Extend source address to accept other protocols. urllib can work with
http(s), file, ftp protocols, so this sould work, but type annotations
are not correct for the latter two. And due to enforcement of the https for
secure connections protocol selection options are still limited.