Prototype changes for multitree

This change contains a prototype implementation for multitree. Several
interfaces and modules are added.

1. Imported/Exported
Modules implementing Exportable interface can export artifacts to other
components. "imported_filegroup" modules can import generated artifacts from other exported modules.

2. Multitree metadata
It contains information about imported/exported modules in each
component, and can be generated via "m update-meta".

3. cc library stub
It's based on prototype stub libraries. It uses imported/exported
mechanism to expose a C API, with a map.txt file and header files.

Bug: 230448564
Test: m
Change-Id: Id7ff7618e2c630c5617a564d8b23b60a1cc9c8e8