Replace 'LOCATE' and '~' module path rewriting with 'locate://' and 'pkg://'

[nlundquist]

New syntax originating from stealjs/steal-less#11 , though that issue is specific to LESS.

The plan is to replace the existing LOCATE and ~ path rewriting syntax with new path schemes that can accomplish more and have more familiar syntax.

'locate://' will work like the existing syntax, importing resources from a module ('bootstrap/main.less') or relative to the file containing the import ('../shared/main.less', './style.less').

'pkg://' will also import resources from a module, but will do relative imports relative to the project root.

[nlundquist]

pkg:// syntax should be relative to npm package root. can't complete that feature as the package path currently isn't exposed through the loader. will add the feature once that path is exposed by modifications to the npm extension.