crate_universe: move build-script dep to extra_deps (#4090)

Fixes #4089.

A crate with a build script depends on its own `build_script_build`
target. That edge was stored in `common_attrs.deps`, which
`Context::workspace_member_deps()` collects to emit crate-hub aliases —
so a library crate with a build script got a dead crate-hub alias for
itself.

Store the edge in `common_attrs.extra_deps` instead:
`workspace_member_deps()` does not read `extra_deps`, so the
self-reference no longer yields an alias. (`extra_deps` holds resolved
labels, so the edge becomes a relative `:build_script_build` — same
target, same package.)

Co-authored-by: UebelAndre <github@uebelandre.com>