Add support for Hatch's {root:uri} paths in editable installs (#2492)

## Summary

If a package uses Hatch's `root.uri` feature, we currently error:

```toml
dependencies = [
  "black @ {root:uri}/../black_editable"
]
```

Even though we're using PEP 517 hooks to get the metadata, which
_should_ support this. The problem is that we load the full
`PyProjectToml`, which means we parse the requirements, which means we
reject what looks like a relative URL in dependencies.

Instead, we should only enforce a limited subset of `pyproject.toml`
(arguably none).

Closes https://github.com/astral-sh/uv/issues/2475.

scripts/editable-installs/root_editable/pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[project]
+name = "root-editable"
+description = 'A simple editable package with a {root:uri} dependency.'
+readme = "README.md"
+requires-python = ">=3.7"
+license = "MIT"
+keywords = []
+authors = [
+  { name = "Astral Software Inc.", email = "hey@astral.sh" },
+]
+classifiers = []
+dependencies = [
+  "black @ {root:uri}/../black_editable"
+]
+version = "0.1.0"
+
+[tool.hatch.metadata]
+allow-direct-references = true

scripts/editable-installs/root_editable/root_editable/__init__.py

@@ -0,0 +1,2 @@
+def func():
+    pass