scan Manifest files in ebuild overlays

We only want DIST lines in Manifest files, so add a check to scan
for it.  Devs that do custom imports often forget.

BUG=None
TEST=manual with custom Manifest files (good and bad)

Change-Id: I277e434e871c36f3b914f2076179fbf542aa46d0
Reviewed-on: https://chromium-review.googlesource.com/66762
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed