If -s is specified, each file is considered a separate
package even if multiple files have the same package names.

For instance, the action and flag "errorcheckdir -s"
will compile all files in the respective directory as
individual packages.

Change-Id: Ic5c2f9e915a669433f66c2d3fe0ac068227a502f
Reviewed-on: https://go-review.googlesource.com/24313
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>