Convert namespace feature to use AST

[jrburke]

Right now the namespace feature just works via some regexp applications, and ideally this would be an AST transform instead. It would help with issues like requirejs/requirejs#1438, where the regexp does not have enough context.

Ideally the solution for this avoids too many conversions to ASTs, just do a one pass of an AST.

Because of that bigger change, likely will not happen for 2.x series, could be something 3.0 or later.