Add support for root file shortcuts in a cookbook like recipe.rb

Also unifies the base name resolution between CookbookVersion and CookbookCompiler for attribute file loading and slightly de-special-cases attribute_filenames= and recipe_filenames=.

The duplicate mappings in CookbookVersion in CookbookVersionLoader are annoying, but the values are just slightly different on either side re: plurals and whatnot so I can't simply invert one to get the other.
Add an explanatory comment.
Additional tests for CookbookVersionLoader decoding root shortcuts.
Wat.
Add host-fqdn to the error message too.
Pass the root_dir in for the test.
Missed the last of these calls.
Rework template/file loading to not require the default/ and to accept an array for explicit search path manipulation. This is the backwards compatible solution to migrate away from the implicit search path in favor of making it explicit when needed (which is rarely).
Tests for new loader behavior.