Update go build constraints

[kaufers]

• Break out providers into unique files so that individual ones can be included in a build
• Group all providers into a common provider.go (with the exception of libvirt and ucs)
• Group all builtin plugins into a common builtin.go

[codecov[bot]]

Codecov Report

Merging #872 into master will decrease coverage by 0.08%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
- Coverage   48.84%   48.76%   -0.09%     
==========================================
  Files          91       91              
  Lines        8213     8213              
==========================================
- Hits         4012     4005       -7     
- Misses       3832     3837       +5     
- Partials      369      371       +2

Impacted Files	Coverage Δ
pkg/rpc/mux/server.go	42.7% <0%> (-5.21%)	:arrow_down:
pkg/types/spec.go	58.4% <0%> (-1.77%)	:arrow_down:

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[Vanuan]

Why is libvirt excluded from builtin providers?

[Vanuan]

Found it: #713 #718