Also, separate type-checking logic into its own file.
go/packages returns import cycle errors anyway, so we just return them instead.
Change-Id: I1f524cdf81e1f9655c1b0afd50dd2aeaa167bb2f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/165021
Reviewed-by: Michael Matloob <matloob@golang.org>
