Define a plugin compiler.

[monopole]

This will better isolate changes from tests as we switch over to modules to better control plugin compatibility assurances. Also, makes it easier to make a compile command.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: monopole

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [monopole]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Liujingfang1]

@monopole Is this compiler only intended to use in testing?

[monopole]

For now, yes, But it needs its own test.
I think we'll need plugin commands like kustomize plugin compile/audit/etc so will use it there too.

[monopole]

fixed some comments.

the goal here is not to make a compiler, but to isolate somewhat complex test infrastructure code that has a particular purpose into a package more appropriate than pkg/target, and place it under test.

but it's not used in any commands yet.

[Liujingfang1]

/lgtm