…ust (bazel-contrib#2871)

Summary:
- Make the requirement line the same as the one that is used in whls. It
  only contains extras and the version if it is present.
- Add debug log statements if we fail to get the version from a direct
  URL reference.
- Move some tests from `parse_requirements_tests` to
  `index_sources_tests` to improve test
  maintenance.
- Replace the URL encoded `+` to a regular `+` in the filename.
- Correctly handle the case when the `=sha256:` is used in the URL.

Once this is merged I plan to tackle bazel-contrib#2648 by changing the
`parse_requirements` code to de-duplicate entries returned
by the `parse_requirements` function.

I cannot think of anything else that we can do for this as of now, so
will mark the associated issue as resolved.

Fixes bazel-contrib#2363
Work towards bazel-contrib#2648

…2931)

The modeling of the data structures returned by the `parse_requirements`
function was not optimal and this was because historically there was
more logic in the `extension.bzl` and more things were decided there.

With the recent refactors it is possible to have a harder to misuse data
structure from the `parse_requirements`. For each `package` we will
return a struct which will have a `srcs` field that will contain easy to
consume values.

With this in place we can do the fix that is outlined in the referenced
issue.

Work towards bazel-contrib#2648

…2932)

This implements the actual fix where we are aggregating the whls and
sdists
correctly from multiple different requirements lines.

Fixes bazel-contrib#2648.
Closes bazel-contrib#2658.