Add buildpack-skipdetect build template

[scothis]

The buildpack-skipdetect build template is a derivative of the buildpack build template that adds configuration support for:

• specifying the buildpack when detection fails
• multi-stage buildpacks
• custom buildpacks not preregistered with packs

In the long run, this build template and the buildpack build template should merge into a single configurable build template.

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[google-prow-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: scothis To fully approve this pull request, please assign additional approvers. We suggest the following additional approver: imjasonh

Assign the PR to them by writing /assign @imjasonh in a comment when ready.

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[scothis]

/assign @imjasonh @sclevine

[googlebot]

CLAs look good, thanks!

[scothis]

@ImJasonH addressed your feedback. Looks like there's no good link for multi-buildpack support from a perspective meaningful for elafros users, so I added a blurb inline. The best we came up with was https://docs.cloudfoundry.org/buildpacks/understand-buildpacks.html which is for buildpack authors.

[scothis]

@sclevine @ImJasonH This PR is ready for a fresh review. The implementation has shifted quite a bit since the original PR. Would you prefer to keep the history or squash the commits into something more meaningful?