Support directories in `uv.tool.cache-keys`

[vivienm]

I work on projects using both Python and Rust. I'd like to make uv rebuild the project when the Rust code is modified, typically when any *.rs file in the src directory is modified.

The new configuration entry uv.tool.cache-keys is promising, but doesn't seem to work with directories.

Possibly related: #2152.

[charliermarsh]

I've been hesitant to add this because in some cases it may just be slower than building the project. Perhaps not for an extension module like that, though.

[vivienm]

Also, it looks like entries such as { file = "some-directory" } are silently ignored. Maybe it could be nice to have a warning?

[sbidoul]

Related: the ask for glob patterns in cache-keys, discussed in #7136