Only use relative paths in lockfile (#6490)

For users who were using absolute paths in the `pyproject.toml`
previously, this is a behavior change: We now convert all absolute paths
in `path` entries to relative paths. Since i assume that no-one relies
on absolute path in their lockfiles - they are intended to be portable -
I'm tagging this as a bugfix.

Closes https://github.com/astral-sh/uv/pull/6438
Fixes https://github.com/astral-sh/uv/issues/6371

crates/uv-requirements/src/source_tree.rs

@@ -171,7 +171,6 @@ impl<'a, Context: BuildContext> SourceTreeResolver<'a, Context> {
         let source = SourceUrl::Directory(DirectorySourceUrl {
             url: &url,
             install_path: Cow::Borrowed(source_tree),
-            lock_path: Cow::Borrowed(source_tree),
             editable: false,
         });