cmd/gofmt, go/format: refactor common pieces into internal/format

cmd/gofmt, go/format: refactor common pieces into internal/format

cmd/gofmt and go/format had 3 functions (parse, format and isSpace)
that had to be kept in-sync.

This CL extracts these 3 functions and refactors them into a new
internal/format package.
This CL is just code reorganization with no behavior nor semantic
change.

Change-Id: I593f24e9d3cadbbd9559a67e3b1d2ff190b4fd90
Reviewed-on: https://go-review.googlesource.com/6760
Reviewed-by: Robert Griesemer <gri@golang.org>