prosody: Fix loading local code when also installed system-wide loader.lua is responsible for adjusting the Lua module loading behavior to account for legacy code (mostly community modules) that don't load resources with the 'prosody' namespace prefix, like `require"prosody.util.json"` rather than `require"util.json"`, as well as the opposite problem of finding resources with the namespace prefix in a local source directory, where Lua would look for `./prosody/util/json.lua` instead of `./util/json.lua`. Which of these two behaviors is active depends on whether the loader is loaded as `require"prosody.loader"` or `require"loader"`, the former being intended for system-wide installations and the later for running from source. When you have a system-wide Prosody installed to the standard Lua search paths, the first attempt to require "prosody.loader" would succeed, leading to system-wide resources being preferred. This makes it difficult to run Prosody from source, e.g. a different version or with local patches, as those changes would be ignored. This change should make it more likely that the loader is invoked as just "loader" instead of "prosody.loader", which determines