Handle global variables in namespaces correctly.

PHP does not support namespaced variables. Variable declarations
inside a namespace are just global declarations.

CCBUG: 255537
REVIEW: 115117
Fix support for PHP namespaces.

Make sure use declarations actually find their original declarations
and implement lookup for definitions within the same namespace but
in a different file.
Also handle name conflicts.

CCBUG: 255537
CCBUG: 244040
REVIEW: 115158