Remove special-casing for editable requirements (#3869)

## Summary

There are a few behavior changes in here:

- We now enforce `--require-hashes` for editables, like pip. So if you
use `--require-hashes` with an editable requirement, we'll reject it. I
could change this if it seems off.
- We now treat source tree requirements, editable or not (e.g., both `-e
./black` and `./black`) as if `--refresh` is always enabled. This
doesn't mean that we _always_ rebuild them; but if you pass
`--reinstall`, then yes, we always rebuild them. I think this is an
improvement and is close to how editables work today.

Closes #3844.

Closes #2695.

crates/uv-requirements/src/source_tree.rs

@@ -104,6 +104,7 @@ impl<'a, Context: BuildContext> SourceTreeResolver<'a, Context> {
         let source = SourceUrl::Directory(DirectorySourceUrl {
             url: &url,
             path: Cow::Borrowed(source_tree),
+            editable: false,
         });
 
         // Determine the hash policy. Since we don't have a package name, we perform a