ExternalData: Allow DATA{} syntax to reference directories

Use a trailing slash to reference a directory.  Require that a list
of associated files be specified to select from within the directory.
One may simply use DATA{Dir/,REGEX:.*} to reference all files but
get a directory passed on the command line.

Tests/Module/ExternalData/Directory/A.dat.md5

@@ -0,0 +1 @@
+9d980b06c2f0fec3d4872d68175b9822

Tests/Module/ExternalData/Directory/B.dat.md5

@@ -0,0 +1 @@
+8f4add4581551facf27237e6577fd662

Tests/Module/ExternalData/Directory/C.dat.md5

@@ -0,0 +1 @@
+c1030719c95f3435d8abc39c0d442946