gccrs: import: Change package opening prototypes

gccrs: import: Change package opening prototypes

Also return a vector of proc macros when trying to open an external
crate.

gcc/rust/ChangeLog:

	* metadata/rust-imports.cc (add_search_path): Change prototype,
	now return a pair of Stream and procedural macro vector.
	(Import::open_package): Likewise.
	(Import::try_package_in_directory): Likewise.
	* metadata/rust-imports.h: Likewise.
	* rust-session-manager.cc (Session::load_extern_crate):
	Likewise.

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