uv pip install -U will downgrade already-installed packages

[charliermarsh]

If you uv init packaging, then change the version to 1000 and install it, then run uv pip install -U black, we'll actually downgrade packaging unnecessarily:

❯ uv pip install -U black
Resolved 6 packages in 93ms
Prepared 5 packages in 0.25ms
Uninstalled 1 package in 0.62ms
Installed 5 packages in 5ms
 + black==24.10.0
 + mypy-extensions==1.0.0
 - packaging==1000 (from file:///Users/crmarsh/workspace/puffin/packaging)
 + packaging==24.2
 + pathspec==0.12.1
 + platformdirs==4.3.6