Allow omitting buildpack version when using pack builder create --flatten

[edmorley]

Description

Currently the new --flatten feature of pack builder create is a bit awkward to use, since it requires that the list of passed buildpacks include the exact buildpack version. For example --flatten 'foo/bar@1.2.3,foo/baz@4.5.6'. If the @ version suffix is omitted, an error is shown.

This requires the use of additional tooling to parse the builder.toml file, extract the versions, and use that to construct a --flatten string.

Proposed solution

Allow omitting the version entirely - ie: `--flatten 'foo/bar,foo/baz' - and if that syntax is used, then the result would be that all buildpacks of that ID would match, regardless of their version.

Describe alternatives you've considered

Allow using a wildcard version - ie: --flatten 'foo/bar@*,foo/baz@*' (which style-wise would match the buildpack version syntax supported by project.toml)

Additional context

• This feature should be documented somewhere