Support built themes and other external css correctly

[agubler]

In order to support built themes and other external CSS we need to ensure we are not processing imports through postcss.

CSS imports always need to contain the extension and must use a tilde prefix when referencing from node_modules:

@import '~@dojo/theme/dojo/index.css'

[codecov]

Codecov Report

Merging #179 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #179   +/-   ##
=======================================
  Coverage   54.72%   54.72%           
=======================================
  Files          10       10           
  Lines         550      550           
  Branches      129      129           
=======================================
  Hits          301      301           
  Misses        249      249

Impacted Files	Coverage Δ
src/base.config.ts	26.08% <ø> (ø)	⬆️

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