* The package spec's reference to the entrypoint package was pointing to
  the hosted cache rather than the package's directory. This was caused
  by the package graph's instance of the entrypoint package incorrectly
  pointing to the cache; the graph now re-uses `entrypoint.root`.

* The package spec didn't contain relative paths for relative path
  dependencies. This was being missed by our tests because the package
  spec parser was automatically converting all paths to absolute at
  parse-time.

Closes #1294

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1228093003 .