[chore] Refactor service/host into service/internal/graph

[TylerHelmuth]

Description

This is a prep PR to reduce the size of #10777. As part of the work to make our component.Host implementation implement componentstatus.Reporter (see #10852), the host struct and graph logic need to be closer together. This is because, as part of #10777 StartAll is changed to depend on our specific Host type instead of a component.Host. Our host already has a dependency on graph, so it can't be moved into its own module.

Link to tracking issue

Related to #10777
Related to #10413

[codecov]

Codecov Report

Attention: Patch coverage is 72.91667% with 26 lines in your changes missing coverage. Please review.

Project coverage is 91.64%. Comparing base (643c17e) to head (c12fd05).

Files	Patch %	Lines
service/internal/graph/host.go	75.32%	19 Missing ⚠️
service/service.go	63.15%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10854   +/-   ##
=======================================
  Coverage   91.64%   91.64%           
=======================================
  Files         406      405    -1     
  Lines       19001    19001           
=======================================
  Hits        17414    17414           
  Misses       1227     1227           
  Partials      360      360           

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