This allows one to download the uv binaries from private mirrors.

The plumbing of the auth attrs allows us to correctly use the `~/.netrc`
or the credential helper for downloading from mirrors that require
authentication.

Testing notes:
* When I tested this, it seems that the dist manifest json may not work
  with private mirrors, but I think it is fine for users in such cases
  to define the `uv` srcs using the `urls` attribute.

Work towards bazel-contrib#1975.

…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.

…st (bazel-contrib#2941)

Bazel does not pass environment variables implicitly (even running test
outside of sandbox). This forces compile_pip_requirements test to fail
with timeout when attempting to run it behind the proxy. Also changes
test_command in dependency_resolver string helper to use dot instead of
underscore following deprecation notice