Fix for API and URL update to go.exp

code.google.com/p/go.exp/go/types -> code.google.com/p/go.tools/go/types
code.google.com/p/go.exp/go/exact -> code.google.com/p/go.tools/go/exact

Also, the API of go/types has changed, so this patch allows errcheck
to compile. The code appears to work correctly, but I'm not 100% sure,
and there are some FIXMEs that should be resolved.