use importer instead of gcimporter, to not only gc-generated file, but also go source 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.
use go1.2 isComparable code, to support go.tools for go1.2