R1:0795cd4baa88

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........................................................