chore(distributor): cleanup of package structure

[warber]

The distributor code did not follow a clean package structure.
This PR tries to improve on this. Packages are now much smaller and semantically divided as much as possible:

Moreover I did a bit of renaming and enhanced the unit tests to use random ports when starting the embedded NATS server.

[codecov]

Codecov Report

Merging #7028 (0c56879) into master (d596efb) will decrease coverage by 0.07%.
The diff coverage is 78.37%.

@@            Coverage Diff             @@
##           master    #7028      +/-   ##
==========================================
- Coverage   57.74%   57.67%   -0.08%     
==========================================
  Files         511      511              
  Lines       29547    29544       -3     
  Branches     1378     1378              
==========================================
- Hits        17063    17039      -24     
- Misses      11252    11272      +20     
- Partials     1232     1233       +1     

Impacted Files	Coverage Δ
distributor/pkg/clientget/client.go	78.57% <ø> (ø)
distributor/pkg/utils/utils.go	80.98% <60.00%> (ø)
distributor/pkg/forwarder/forwarder.go	47.09% <66.66%> (ø)
distributor/pkg/uniform/log/uniformlog.go	71.87% <66.66%> (ø)
distributor/pkg/poller/poller.go	68.88% <85.71%> (ø)
distributor/pkg/receiver/receiver.go	61.01% <90.00%> (ø)
...utor/pkg/natsconnection/nats_connection_handler.go	77.77% <100.00%> (ø)
...stributor/pkg/uniform/controlplane/controlplane.go	97.64% <100.00%> (ø)
distributor/pkg/uniform/watch/uniformwatch.go	60.00% <100.00%> (ø)
... and 1 more

Flag	Coverage Δ
distributor	70.17% <78.37%> (-2.04%)	⬇️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[RealAnna]

LGTM