Preserve conflicting mutators

[willbeason]

Preserve conflicting mutator paths, making behavior independent of the order in which conflicting mutators are added.

Increase test coverage from 70% to 89%. Added tests are unit tests that fake out the dependencies of Reconciler, so they don't require the same setup as the existing integration tests in the same package.

Minor changes included in this PR:

• Make pod ownership mode a member of Reconciler rather than a global variable. As on master makes testing both paths consistently impossible in unit tests, but this way makes the tests independent of which order they were run (or whether other unit tests were run). It is still a global variable, but it is only read when creating a Reconciler via newReconciler().
• Add Type to MutatorError for machine consumption. This allows for selectively replacing errors or marking "enforced" in the case that a conflict between mutators is resolved, but one of the mutators has a separate error as well.
• Cleanup POD_NAMESPACE environment variable after unit tests which set it to make unit tests independent.
• OPA clients named opa renamed to opaClient to prevent conflicts between the package name and variables with the name.
• Split Reconciler and Adder code into their own files (respectively named adder.go and reconciler.go). This reduces the file size and makes finding relevant code easier.
• Delete SetupTestReconciler since it isn't used by anything.
• Implement statusUpdate which is a functor that updates MutatorPodStatus
• Make MutatorIngestionStatus enum values into constants to prevent them from being mistakenly reassigned.
• Make ErrConflictingSchema a structured error type which records the IDs of the Mutators which conflict. This allows just accessing the set of conflicting mutators via erorrs.As elsewhere.
• Add GetConflicts() to schema.node to dynamically fetch the conflicts a given mutator has with all other mutators.
• Update system_test tests which validate that mutator conflicts are now handled as we expect.

Fixes: #1216

[codecov-commenter]

Codecov Report

Merging #1569 (82f40fa) into master (921869d) will increase coverage by 0.68%.
The diff coverage is 76.54%.

@@            Coverage Diff             @@
##           master    #1569      +/-   ##
==========================================
+ Coverage   52.83%   53.52%   +0.68%     
==========================================
  Files          89       93       +4     
  Lines        7849     7995     +146     
==========================================
+ Hits         4147     4279     +132     
- Misses       3356     3372      +16     
+ Partials      346      344       -2     

Flag	Coverage Δ
unittests	53.52% <76.54%> (+0.68%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/controller/constraint/constraint_controller.go	5.68% <0.00%> (-0.04%)	⬇️
pkg/controller/mutators/stats_reporter.go	70.21% <ø> (ø)
pkg/readiness/ready_tracker.go	69.82% <0.00%> (-0.40%)	⬇️
pkg/mutation/schema/errors.go	33.33% <33.33%> (ø)
pkg/mutation/schema/schema.go	80.76% <33.33%> (-10.73%)	⬇️
pkg/mutation/system.go	83.44% <50.00%> (-7.40%)	⬇️
pkg/controller/mutators/core/adder.go	53.12% <53.12%> (ø)
pkg/mutation/schema/id_set.go	63.15% <63.15%> (ø)
pkg/mutation/schema/node.go	89.80% <92.59%> (+0.33%)	⬆️
pkg/controller/mutators/core/reconciler.go	85.71% <93.44%> (ø)
... and 15 more

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[maxsmythe]

LGTM, couple nits