Add cycle detection to celerity mapper

Summary: create CelerityResourceGraph, which extends AbstractDirectedGraph.
since we've done a bunch of work already to load the resource graph into memory
CelerityResourceGraph simply stores a copy and makes loadEdges work off that
stored copy.

Test Plan:
made phabricator-prefab require herald-rule-editor

~/code/phabricator> ./scripts/celerity_mapper.php webroot
Finding static resources...
Processing 154
files..........................................................................................................................................................
[2011-11-22 11:28:29] EXCEPTION: (Exception) Cycle detected in resource graph:
phabricator-prefab => herald-rule-editor => phabricator-prefab at
[/Users/btrahan/Dropbox/code/phabricator/scripts/celerity_mapper.php:173]

fixed phabricator-prefab requiring herald-rule-editor. re-ran celerity_mapper
and no errors!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Differential Revision: 1132