[foreign-memaccess] RFR: Flatten package hierarchy

[Maurizio Cimadamore]

Given the small number of classes/interfaces involved in this API, I 
think it would preferrable to use a single package (java.foreign).

This choice will also be resilient for when we do the later ABI layer, 
which only introduces one public SystemABI interface.

For higher-level binder/jextract stuff, I think it makes sense to use a 
separate package (java.foreign.binder?)

http://cr.openjdk.java.net/~mcimadamore/panama/8223614/

Maurizio