gccrs: Generalize prelude import handling

Replace some ad-hoc handling of prelude imports with a glob import in
every module/crate.

gcc/rust/ChangeLog:

	* resolve/rust-default-resolver.cc
	(DefaultResolver::visit (Crate)): Call maybe_prelude_import.
	(DefaultResolver::visit (Module)): Likewise.
	* resolve/rust-default-resolver.h
	(DefaultResolver::maybe_prelude_import): New virtual member
	function.
	* resolve/rust-early-name-resolver-2.0.cc
	(Early::maybe_prelude_import): New member function definition.
	(Early::finalize_glob_import): Mark visitor as dirty when
	first setting ctx.prelude.
	* resolve/rust-early-name-resolver-2.0.h
	(Early::maybe_prelude_import): New member function declaration.
	* resolve/rust-late-name-resolver-2.0.cc
	(Late::visit (IdentifierExpr)): Remove ad-hoc prelude import
	handling.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>