cargotest/support: remove internal mutability in favor of switching types

cargotest/support: remove internal mutability in favor of switching types

Remove is_build: Cell<bool> from ProjectBuilder and introduce a new type Project.

is_build==false <-> ProjectBuilder
is_build==true <-> Project

Also add #[must_use] to ProjectBuilder to confirm its instances are surely consumed by its build() method to produce Project.

The same goes for RepoBuilder.

ProjectBuilder::cargo_process() was removed as its design heavily depended on the internal mutability.

Signed-off-by: NODA, Kai <nodakai@gmail.com>