resourceloader: Deduplicate module registration conflicts
If an attempt is made to register a ResourceLoader module which has already
been registered, instead of bailing with an error, issue a warning instead, and
allow the lattermost set of module parameters to stick.
Because modules are registered from extensions, and because modules are often
migrated from one extension to another as extensions are refactored, and
because the developers of different extensions are not always aware of each
other's work, throwing an error runs a high risk of causing an outage.