rust: Resolve crate roots outside rust-project

Previously, we manually re-computed crate roots inside project_json for
rendering rust-project.json. In addition to added complexity, this meant
that generated sources and glob sources would not render correctly - it
would select e.g. `src/**.rs` or `:foo` as a crate root.

Use a centralized computation of crate roots instead.

Bug: 309943184
Test: SOONG_GEN_RUST_PROJECT=1 m nothing, compare rust-project.json
Change-Id: I0caddbf600d025a0041f45e69812cdd6f1761234