Move fhirig into it's own Go library

[bitwizeshift]

The internal/fhirig package connects to the simplifier.net registry of FHIR packages and is able to download all dependencies recursively for a given IG definition. Currently this is used internally just so that we can fetch all definitional resources needed for generating bindings; but realistically there is no reason why this can't be shared across other tools, since it's functionality is logically independent and can be used in other things.

Ideally this should be in its own new Go library (perhaps a package, go-fhirig?). This ticket is to track this idea for the future.

[github-actions]

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[bitwizeshift]

Update: Several recent reworkings have introduced some of this functionality into exported sub-packages of this project (fhenix/registry) . I still haven't decided whether it belongs in a whole separate library, vs just as a reusable component from part of this library.

[fhir-bellows]

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