go/types: move gotype command into this directory

go/types: move gotype command into this directory

This is a copy of x/tools/cmd/gotype/gotype.go with the corresponding
x/tools/cmd/gotype/doc.go prepended and including a build tag (ignore).

This way, go/types can be built unaffected. If we need the gotype command,
it is trivially built in the go/types directory with: go build gotype.go .

Fixes #12303.

Change-Id: I2d792fcb39719cc5cc300f657e4735901cd20faa
Reviewed-on: https://go-review.googlesource.com/15152
Reviewed-by: Rob Pike <r@golang.org>