pack build --buildpack accepts a buildpackage as an argument

[jromero]

If you have an image for a buildpackage on a registry, that can be passed in the --buildpack flag during pack build

Acceptance criteria:

Given I have access to a  buildpackage on a registry or your local docker daemon
When you run `pack build MY_APP --buildpack <BUILDPACKAGE:tag>`
Then the build executes using the packaged buildpacks using the current default builder

Given I have access to a  buildpackage on a registry or your local docker daemon
When you run `pack build MY_APP --buildpack <BUILDPACKAGE:tag> --buildpack <BUILDPACKAGE:tag>`
Then the build executes using the packaged buildpacks follow the existing semantics


Given I have access to a  buildpackage on a registry or your local docker daemon
When you run `pack build MY_APP --buildpack <BUILDPACKAGE@sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855>`
Then the build executes using the packaged buildpacks follow the existing semantics

Note:

• This issue does not include support for .cnb files.
• This issue only initially tackles building from unambigous buildpackages, IE repo/image:tag. Later story will support disambiguating.