refactor: move util.Wildcard into its own package

[xrstf]

What this PR does / why we need it:
We ran into an issue when updating our product to controller-runtime 0.15. We tried gatekeeper v3.13.0-beta.1 and ended up with

github.com/open-policy-agent/gatekeeper/v3/pkg/util
# github.com/open-policy-agent/gatekeeper/v3/pkg/util
../../github.com/open-policy-agent/gatekeeper/pkg/util/pack.go:44:9: cannot use func(obj client.Object) []reconcile.Request {…} (value of type func(obj client.Object) []reconcile.Request) as handler.MapFunc value in return statement
../../github.com/open-policy-agent/gatekeeper/pkg/util/pack.go:70:9: cannot use func(obj client.Object) []reconcile.Request {…} (value of type func(obj client.Object) []reconcile.Request) as handler.MapFunc value in return statement
../../github.com/open-policy-agent/gatekeeper/pkg/util/pack.go:75:13: not enough arguments in call to mf
        have (*unstructured.Unstructured)
        want (context.Context, client.Object)

This is because the signature for MapFuncs has changed to include a context.Context now.

To solve this issue, this PR moves the util.Wildcard type into its own package (analogous to pkg/version). This removes the dependency from apis/config/v1alpha1 to pkg/util, thereby removing the need to compile-in a full-blown controller-runtime library for anyone who wants to use Gatekeeper's config/v1alpha1 API.

[maxsmythe]

LGTM, thanks for the PR!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.48%. Comparing base (d8f2bbc) to head (8c1a529).
⚠️ Report is 811 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/controller/config/process/excluder.go	25.00%	3 Missing ⚠️
pkg/mutation/match/zz_generated.deepcopy.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2853      +/-   ##
==========================================
- Coverage   53.55%   53.48%   -0.07%     
==========================================
  Files         133      133              
  Lines       11536    11536              
==========================================
- Hits         6178     6170       -8     
- Misses       4882     4888       +6     
- Partials      476      478       +2     

Flag	Coverage Δ
unittests	53.48% <16.66%> (-0.07%)	⬇️

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