gccrs: Change lookup_module function return type

gccrs: Change lookup_module function return type

Wrap the return type into an optional.

gcc/rust/ChangeLog:

	* checks/errors/privacy/rust-visibility-resolver.cc: Update function
	call to match the new return type.
	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::resolve_root_path):
	Likewise.
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::resolve_root_path):
	Likewise.
	* util/rust-hir-map.cc (Mappings::insert_module): Likewise.
	(Mappings::lookup_module): Change the function's return type.
	* util/rust-hir-map.h: Update the function's prototype.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>