Deprecate Metadata for Generating Module Prefixes in Mapped Code

[InsertCreativityHere]

We have a handful of metadata which are about re-mapping top-level modules:
java:package, cs:namespace, swift:module, maybe even more in other languages.

Confusingly, when these are applied to a module, it doesn't change the module's name itself.
Instead it just changed which scope the module's definition will be mapped into.

["cs:namespace:Something"]
module Hello {...}

Will cause us to generate code into the Something.Hello namespace.

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Now that we've settled on introducing other metadata for remapping identifiers, these metadata are redundant, and should be deprecated.