Unit test bundle for scope and identifiers about hlb

[antonio-osorio]

@hinshun I would like to move import testing out of this ticket. The way that it is implemented right now it communicates with the buildkit client even for local files so the testing is a bit more involved.

[hinshun]

@antonio-osorio You're right, we should of moved it over in the first place, sorry. I think the compile_test.go really should be just testing the output of checker.Check and not the full Compile. I moved compile_test.go to checker/checker_test.go and simplified the test. I added an import test there, could you please port your test cases over this: #96