Move internal functions into internal packages

[lbernick]

Based on this discussion thread.
Everything in the apis packages and the client packages should be public, and all other code should be moved into the internal folder where possible to avoid usage outside of the repo and reduce our Go API surface.

[vdemeester]

Everything in the apis packages and the client packages should be public, and all other code should be moved into the internal folder where possible to avoid usage outside of the repo and reduce our Go API surface.

Note that today, other tektoncd component might depend on those, for example the cli, or the operator, or triggers or even chains. I think for each package we want to move we need to see if it make sense or not to be "public" (and exported) or not.

See #5681 (review)

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[vdemeester]

/lifecycle frozen