Move sources to /internal and rename some APIs

[chrisdoherty4]

Move sources under /internal so its clear nothing in this repository is intended for import. Additionally tidy the package API for /internal/xff to simplify calls.

[codecov]

Codecov Report

Merging #134 (252ae11) into main (6ea659c) will decrease coverage by 0.07%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   24.26%   24.19%   -0.08%     
==========================================
  Files           7        7              
  Lines         680      682       +2     
==========================================
  Hits          165      165              
- Misses        493      494       +1     
- Partials       22       23       +1     

Impacted Files	Coverage Δ
internal/hardware/client.go	0.00% <0.00%> (ø)
internal/hardware/export.go	0.00% <ø> (ø)
internal/hardware/tink.go	0.00% <ø> (ø)
internal/http/api.go	0.00% <ø> (ø)
internal/http/handlers.go	55.10% <ø> (ø)
internal/hardware/kubernetes.go	40.16% <33.33%> (ø)
internal/http/server.go	74.46% <50.00%> (ø)

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