Enable package overrides in pip-compile

[charliermarsh]

Summary

This PR enables overrides to be passed to pip-compile and pip-install via a new --overrides flag.

When overrides are provided, we effectively replace any requirements that are overridden with the overridden versions. This is applied at all depths of the tree.

The merge semantics are such that we replace all requirements of a package with all requirements from the overrides files. So, for example, if a package declares:

foo >= 1.0; python_version < '3.11'
foo < 1.0; python_version >= '3.11'

And the user provides an override like:

foo >= 2.0

Then both of the foo requirements in the package will be replaced with the override.

If instead, the user provided an override like:

foo >= 2.0; python_version < '3.11'
foo < 3.0; python_version >= '3.11'

Then we'd replace both of the original foo requirements with both of these overrides. (In technical terms, for each package in the requirements file, we flat-map over its overrides.)

Closes #511.