Allow multiple source entries for each package in tool.uv.sources

[charliermarsh]

Summary

This PR enables users to provide multiple source entries in tool.uv.sources, e.g.:

[tool.uv.sources]
httpx = [
  { git = "https://github.com/encode/httpx", tag = "0.27.2", marker = "sys_platform == 'darwin'" },
  { git = "https://github.com/encode/httpx", tag = "0.24.1", marker = "sys_platform == 'linux'" },
]

The implementation is relatively straightforward: when we lower the requirement, we now return an iterator rather than a single requirement. In other words, the above is transformed into two requirements:

httpx @ git+https://github.com/encode/[email protected] ; sys_platform == 'darwin'
httpx @ git+https://github.com/encode/[email protected] ; sys_platform == 'linux'

We verify (at deserialization time) that the markers are non-overlapping.

Closes #3397.