Enable projects to opt-out of workspace management (#4565)

## Summary

You can now add `managed = false` under `[tool.uv]` in a
`pyproject.toml` to explicitly opt out of the project and workspace
APIs.

If a project sets `managed = false`, we will (1) _not_ discover it as a
workspace root, and (2) _not_ discover it as a workspace member (similar
to using `exclude` in the workspace parent).

Closes https://github.com/astral-sh/uv/issues/4551.

scripts/workspaces/albatross-project-in-excluded/packages/seeds/pyproject.toml

@@ -0,0 +1,12 @@
+[project]
+name = "seeds"
+version = "1.0.0"
+requires-python = ">=3.12"
+dependencies = ["idna==3.6"]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.uv]
+managed = false

scripts/workspaces/albatross-project-in-excluded/packages/seeds/src/seeds/__init__.py

@@ -0,0 +1,5 @@
+import idna
+
+
+def seeds():
+    print("sunflower")