The knative.dev/client-pkg/pkg/kn-source-pkg sub-package should be renamed and flatten

[cardil]

The knative.dev/client-pkg/pkg/kn-source-pkg sub-package was introduced in https://github.com/knative/client-pkg/pull/74, but contains some weird stuff, that shouldn't be present in a library:

• a main() file
• some hack scripts
• e2e tests for kn-source-plugin, but (what looks like) name-replaced

Let's clean up this as well, and leave just the actual library, just renamed to knative.dev/client-pkg/plugin/source/....

_Originally posted by @cardil in https://github.com/knative/client-pkg/pull/155#discussion_r1516637050_

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