Add directory `--find-links` support for local paths to pip-compile.
It seems that pip joins all sources and then picks the best package. We
explicitly give find links packages precedence if the same exists on an
index and locally by prefilling the `VersionMap`, otherwise they are
added as another index and the existing rules of precedence apply.
Internally, the feature is called _flat index_, which is more meaningful
than _find links_: We're not looking for links, we're picking up local
directories, and (TBD) support another index format that's just a flat
list of files instead of a nested index.
`RegistryBuiltDist` and `RegistrySourceDist` now use `WheelFilename` and
`SourceDistFilename` respectively. The `File` inside `RegistryBuiltDist`
and `RegistrySourceDist` gained the ability to represent both a url and
a path so that `--find-links` with a url and with a path works the same,
both being locked as `<package_name>@<version>` instead of
`<package_name> @ <url>`. (This is more of a detail, this PR in general
still work if we strip that and have directory find links represented as
`<package_name> @ file:///path/to/file.ext`)
`PrioritizedDistribution` and `FlatIndex` have been moved to locations
where we can use them in the upstack PR.
I added a `scripts/wheels` directory with stripped down wheels to use
for testing.
We're lacking tests for correct tag priority precedence with flat
indexes, i only confirmed this manually since it is not covered in the
pip-compile or pip-sync output.
Closes#876
