Fix an edge case in the dependency computer.

When a package both transformed itself and had a dev transformer (that
is, a transformer that only runs on non-public files), *and* that
package's self-transformed code was imported by another transformer, the
dependency computer could return the dev transformer in its results.

Closes #1290
Closes #1291

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1220223008.