Discover workspaces without using them in resolution (#3585)
Add minimal support for workspace discovery, only used for determining
paths in the bluejay commands.
We can now discover the workspace structure, namely that the
`pyproject.toml` of a package belongs to a workspace `pyproject.toml`
with members and exclusion. The globbing logic is inspired by cargo. We
don't resolve `workspace = true` metadata declarations yet.
