SOLR-5520: Backport of SOLR-4882 (SolrResourceLoader was restricted to only allow access to resource files below the instance dir)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_3_6@1546958 13f79535-47bb-0310-9956-ffa450edef68