Reverts:

- #14349
- #14346
- #14245

Retains the test cases. Includes a `find-links` test case.

Supersedes

- #14387
- #14503

We originally got a report at
#13707 that inclusion of a
trailing slash on an index URL was causing lockfile churn despite having
no semantic meaning and resolved the issue by adding normalization that
stripped trailing slashes at parse time.

We then discovered that, while there are not semantic differences for
trailing slashes on Simple API index URLs, there are differences for
some flat (or find links) indexes. As reported in
#14367, the change in
#14245 caused a regression for at
least one user.

We attempted to fix the regression via a few approaches.

#14387 attempted to differentiate
between Simple API and flat index URL parsing, but failed to account for
the `Deserialize` implementation, which always assumed Simple API-style
index URLs and incorrectly trimmed trailing slashes in various cases
where we deserialized the `IndexUrl` type from a file. I attempted to
resolve this by performing a larger refactor, but it ended up being
quite painful. In particular, the `Index` type was a blocker — we don't
know the `IndexUrl` variant until we've parsed the `IndexFormat` and
having a multi-stage deserializer is not appealing but adding a new
intermediate type (i.e., `RawIndex`) is painful due to the pervasiveness
of `Index`. Given that we've regressed behavior here and there's not a
straight-forward fix, we're reverting the normalization entirely.

#14503 attempted to perform
normalization at compare-time, but that means we'd fail to invalidate
the lockfile when the a trailing slash was added or removed and given
that a trailing slash has semantic meaning for a find-links URL... we'd
have another correctness problem.

After this revert, we'll retain all index URLs verbatim. The downside to
this approach is that we'll be adding a bunch of trailing slashes back
to lockfiles that we previously normalized out, and we'll be reverting
our fix for users with inconsistent trailing slashes on their index
URLs. Users affected by the original motivating issue should use
consistent trailing slashes on their URLs, as they do frequently have
semantic meaning. We may want to revisit normalization and type aware
index URL parsing as part of a larger change.

Closes  #14367