Support import source file, instead of import only gc-generated file

There are 2 changes:
1. Support import source file and gc-generated file, instead of import only gc-generated file
Scenario: If a project proj has two package : proj/a and proj/b, proj/b import proj/a, and proj/a is not “go install”ed.
When use gcimporter only, gen proj/b will failed with “proj/a is not found for import”.
It will work with importer.

The code change is to use customized types.Config to check type, instead of types.Check. In the types.Config, Import will use importer to import libs, including source file and gc-generated files

2. Update predicates.isComparable, to support go.tools for go1.2

Thanks.