Convert link for equivalency check.
This is used in the resolver to check whether two URL-specified requirements
likely point to the same distribution and can be considered equivalent. This
equivalency logic avoids comparing URLs literally, which can be too strict
(e.g. "a=1&b=2" vs "b=2&a=1") and produce conflicts unexpecting to users.
Currently this does three things:
1. Drop the basic auth part. This is technically wrong since a server can
serve different content based on auth, but if it does that, it is even
impossible to guarantee two URLs without auth are equivalent, since
the user can input different auth information when prompted. So the
practical solution is to assume the auth doesn't affect the response.
2. Parse the query to avoid the ordering issue. Note that ordering under the
same key in the query are NOT cleaned; i.e. "a=1&a=2" and "a=2&a=1" are
still considered different.
3. Explicitly drop most of the fragment part, except ``subdirectory=`` and
hash values, since it should have no impact the downloaded content. Note
that this drops the "egg=" part historically used to denote the requested
project (and extras), which is wrong in the strictest sense, but too many
people are supplying it inconsistently to cause superfluous resolution
conflicts, so we choose to also ignore them.