Support for typename import aliases

[TristonianJones]

Allow users to specify type name imports in policies.

Imported types are aliased by the simple type name:

# Create an import alias to `TypeName`
imports:
  - name: my.custom.really.ridiculously.long.package.TypeName

rule:
  match:
    - output: "TypeName{}"

As a future refinement, the imports will support a custom alias to allow
for alias disambiguation between types which come from different packages
but have the same simple name.

[TristonianJones]

FYI @seirl, I've added proper support for this feature.

[TristonianJones]

PTAL