policy: revamped policy merge mechanism

[jkowalski]

Added policy.Definition which allows us to precisely report where
each piece of policy came from.

Fixed a one-off bug with "noParent", which prevented merging of parent
policies one level too soon.

Added a whole bunch of merging helpers and generic reflection-based
test that ensures every single merge is tested.

[codecov]

Codecov Report

Merging #1538 (a929b90) into master (b91d8ac) will increase coverage by 0.20%.
The diff coverage is 89.45%.

@@            Coverage Diff             @@
##           master    #1538      +/-   ##
==========================================
+ Coverage   68.94%   69.15%   +0.20%     
==========================================
  Files         374      375       +1     
  Lines       29657    29639      -18     
==========================================
+ Hits        20447    20496      +49     
+ Misses       7525     7474      -51     
+ Partials     1685     1669      -16     

Impacted Files	Coverage Δ
snapshot/policy/log_detail.go	100.00% <ø> (ø)
snapshot/policy/optional.go	100.00% <ø> (ø)
snapshot/policy/policy.go	88.88% <ø> (+13.41%)	⬆️
snapshot/policy/policy_tree.go	100.00% <ø> (ø)
snapshot/policy/policy_manager.go	72.90% <44.44%> (ø)
cli/command_policy_show.go	57.43% <67.74%> (-3.95%)	⬇️
cli/command_repository_create.go	69.44% <100.00%> (ø)
cli/command_snapshot_list.go	60.26% <100.00%> (ø)
internal/server/api_snapshots.go	83.01% <100.00%> (ø)
internal/server/source_manager.go	75.57% <100.00%> (ø)
... and 25 more

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