go/loader: rename SourceImports flag to ImportFromBinary and invert sense

...since the zero value is more useful by far.

This is a breaking API change, obviously.  (One or two tests in this
CL have intentional been left using the zero value, i.e., they now
load source.)

Change-Id: I42287bfcdb1afef8ee84e5eac12534dd0a1fd5d2
Reviewed-on: https://go-review.googlesource.com/5653
Reviewed-by: Robert Griesemer <gri@golang.org>
20 files changed