gccrs: import: Load procedural macros into external crate

gccrs: import: Load procedural macros into external crate

Load the procedural macro array into the external crate structure.

gcc/rust/ChangeLog:

	* metadata/rust-imports.cc (Import::try_package_in_directory):
	Add call to load macros before loading metadatas.
	* expand/rust-proc-macro.cc: Return empty vector instead of
	panicking on error.

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