Support building lambdas and multiplatform pexes with the uv resolver

[benjyw]

Currently the uv resolver only supports building pexes for the local platform. It doesn't accept complete_platforms, and so cannot build lambdas or multiplatform pexes.

The building blocks for supporting this already exist:

• uv can build foreign platform venvs via the --python-platform flag
• pex can receive multiple --venv-repository arguments.