Add automatic URL mirroring for naclports.

Split out the core package metadata from the package build scripts.
Each package now contains a pkg_info file containing just to
core metadata.  The new update_mirror.py script can then parse
this metadata to keep the mirror up-to-date.  URLS_TO_MIRRORS then
becomes redundant.

Also, each package now specified the official upstream tarball
and the build system will automatically try to download from
commonstorage before falling back to upstream.  This means that
developers can submit working packages that work fine both
before and after the URL has been mirrored.

BUG=None

Review URL: https://codereview.chromium.org/11783074

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@655 7dad1e8b-422e-d2af-fbf5-8013b78bd812