Refactor depends-on code

[karlkfi]

• Move from pkg/object to pkg/object/dependson
• Add DependencySet type to abstract complexity specific to the depends-on annotation (set equality, format/parse)
• Add Parse/Format funcs for ObjMetadata and DependencySet
• Refactor primary API to ReadAnnotation and WriteAnnotation
• Move Write/Format behavior from testutil into dependson pkg
• Make constants private to encourage using functions as primary API

The following optional refactors may make good followup PRs:

• Refactor non-annotation uses of []ObjMetadata to use DependencySet, and replace object.SetEquals with DependencySet.Equal
• Replace object.ObjMetas (used only twice for Contains method) with DependencySet
• Refactor logging to use FormatObjMetadata and FormatDependencySet with a String() wrapper, to replace the %s/%s used in many places.
• Rename ObjMetadata to ResourceReference
• Move ObjMetadata to pkg/object/reference or ResourceReference to pkg/object/reference to reduce package complexity and simplify naming of functions

[k8s-ci-robot]

Hi @karlkfi. Thanks for your PR.

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[seans3]

/ok-to-test

[seans3]

/lgtm

[k8s-ci-robot]

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