NewPackage creates an ast.Package node from
a set of package files and resolves unresolved
identifiers.

Also:
- Changed semantics of Scope.Insert: If an
  object is inserted w/o errors, the result
  is nil (before it was obj).
- Fixed an identifier resolution bug in the
  parser: map keys must not be resolved.

gotype runs through several go/* packages
and successfully resolves all (non-field/method)
identifiers.

R=rog, rsc
CC=golang-dev
https://golang.org/cl/4298044