Recursively allow URL requirements for local dependencies (#2702)
## Summary
This is a trimmed-down version of
https://github.com/astral-sh/uv/pull/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
```
