cmd/compile/internal/types: unexport PkgMap, remove PkgList

cmd/compile/internal/types: unexport PkgMap, remove PkgList

- PkgMap was only needed to test import/export in a "cleanroom"
  environment, with debugFormat set. Provided helper function
  instead.

- PkgList was only used to identify directly imported packages.
  Instead, compute that list explicitly from the package map.
  It happens only once, the list is small, and it's more robust
  than keeping two data structures in sync.

Change-Id: I82dce3c0b5cb816faae58708e877799359c20fcb
Reviewed-on: https://go-review.googlesource.com/41078
Reviewed-by: Matthew Dempsky <mdempsky@google.com>