Move step and sidecar replacements to new package

[lbernick]

Changes

This commit moves logic related to variable replacement in Steps and Sidecars
out of pkg/apis into its own package. This will prevent this code from being
duplicated in the v1 api package. No functional changes.

/kind cleanup

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• n/a Has Docs included if any changes are user facing
• n/a Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• n/a Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• n/a Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/container/container_replacements.go	Do not exist	100.0%
pkg/container/sidecar_replacements.go	Do not exist	100.0%
pkg/container/step_replacements.go	Do not exist	66.7%

[lbernick]

/retest

[lbernick]

/test check-pr-has-kind-label

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[abayer]

/lgtm