Change to use variant's root instead of uri

[davidlatwe]

Problem

If the application package has variants, using uri attribute to format path like {root}/sub-dir is incorrect.

For example (the default is the variant) :

> pkg.uri
/Users/davidlatwe/rez/packages/install/mockmaya/2018/package.py[0]
> pkg.root
/Users/davidlatwe/rez/packages/install/mockmaya/2018/default

Change

Use attribute root when the package object is a variant. ☺️

[davidlatwe]

Merging this today, patch version will be auto bumped.