gocore: convert dwarf names to runtime more accurately

Import paths can have dots, and struct names need to have package paths
removed.

This accounts for import paths like github.com/whatever, and also should
fix things like map[path/to/package.Type]path/to/package.Othertype.

Change-Id: Ief767f7fadf5c15f6d6343e0e1738904a4ca460a
Reviewed-on: https://go-review.googlesource.com/99675
Reviewed-by: Keith Randall <khr@golang.org>