This is a first step towards moving go/types from the tools
repo into the std repo. The files were brought over via the
added src/go/types.bash script for reproducability. The
script can be removed once all dependencies on go/types
have moved to the std repo go/types.

The script moved packages as follows:

- x/tools/go/types => go/types (type-checker)
- x/tools/go/exact => go/exact (constants)
- x/tools/go/gcimporter => go/types/internal/gcimporter

The gcimporter is needed to be able to run tests. go/types
should probably have some factory function to provide an
appropriate importer.

Some of the go/types tests fail for a handful of platforms
(windows and nacl). In order to keep this change "clean"
from manual changes, the next change will disable those
tests for now so we can move forward.

Change-Id: I448d8f7faa39ad2e04811911b699f7682627c224
Reviewed-on: https://go-review.googlesource.com/8530Reviewed-by: Rob Pike <r@golang.org>