Recursively allow URL requirements for local dependencies

[charliermarsh]

Summary

This is a trimmed-down version of #2684 that only applies to local source trees for now, which enables workspace-like workflows (whereby local packages can depend on other local packages at arbitrary depth).

Closes #2699.

Test Plan

Added new tests.

Also cloned this MRE that was shared with me (https://github.com/timothyjlaurent/uv-poetry-monorepo-mre), and verified that it was installed without error:

❯ cargo run pip install ./uv-poetry-monorepo-mre/app --no-cache
    Finished dev [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv pip install ./uv-poetry-monorepo-mre/app --no-cache`
Resolved 4 packages in 1.28s
   Built app @ file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/app
   Built lib1 @ file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/lib1
   Built lib2 @ file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/lib2                                        Downloaded 4 packages in 457ms
Installed 4 packages in 2ms
 + app==0.1.0 (from file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/app)
 + lib1==0.1.0 (from file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/lib1)
 + lib2==0.1.0 (from file:///Users/crmarsh/workspace/uv/uv-poetry-monorepo-mre/lib2)
 + ruff==0.3.4