update __all__ and clean up imports for misc ebuild stuff
ChunkedDataDict: preserve global settings when cloning while unfreezing
ChunkedDataDict: merge global settings onto package specific values

This fixes properly layering use.force, use.stable.force,
package.use.force, and package.use.stable.force (and the similar files
for masking).
fix layering profile files for use masking and forcing

Previously the layering of use.force, use.stable.force,
package.use.force, and package.use.stable.force (and similar masking
files) was wrong, leading to global settings being overridden.
clean up files after tests so following tests aren't altered
add initial tests for stable arch related profile settings

Specifically use.stable.mask, package.use.stable.mask, use.stable.force,
and package.use.stable.force. Also tests for proper layering, i.e.
use.mask -> use.stable.mask -> package.use.mask -> package.use.stable.mask.