File ingestion assumes symlinks are not broken

When files are ingested for a charm, we assume that following a symlink will work, but it does not work if the symlink is broken.

In the example below, tests.py is a symlink to "../../buildbot-master/hooks/tests.py", but that path does not exist.

Not only should we guard against missing symlinks, we should also ensure they do not traverse outside the charm's directory, since that would be a security violation.

2013-04-25 10:55:06,409 ERROR [charm.bzr][MainThread] bzr error on {u'name': u'buildbot-slave', u'series': u'oneiric', 'branch_dir': '/home/abentley/var/charms/oneiric/frankban/buildbot-slave/trunk', u'bname': u'trunk', u'commit': <email address hidden>', u'owner': u'frankban', u'branch_spec': u'~frankban/charms/oneiric/buildbot-slave/trunk', '_id': u'~frankban/charms/oneiric/buildbot-slave/trunk', u'distro_series': []}: [Errno 2] No such file or directory: '/home/abentley/var/charms/oneiric/frankban/buildbot-slave/trunk/hooks/tests.py'
Traceback (most recent call last):
  File "/home/abentley/canonical/charmworld/charmworld/jobs/ingest.py", line 202, in run_job
    job.run(charm_data)
  File "/home/abentley/canonical/charmworld/charmworld/jobs/ingest.py", line 184, in run
    charm_data = self.add_files(charm_data)
  File "/home/abentley/canonical/charmworld/charmworld/jobs/ingest.py", line 117, in add_files
    self.store_branch_files(charm_data)
  File "/home/abentley/canonical/charmworld/charmworld/jobs/ingest.py", line 110, in store_branch_files
    self.fs, charm_data, charm_data['branch_dir'])
  File "/home/abentley/canonical/charmworld/charmworld/models.py", line 367, in save_files
    subdir=directory))
  File "/home/abentley/canonical/charmworld/charmworld/models.py", line 292, in save_file
    with open(file_path) as handle:
IOError: [Errno 2] No such file or directory: '/home/abentley/var/charms/oneiric/frankban/buildbot-slave/trunk/hooks/tests.py'