generate: add bundle subcommand for current project layouts

[estroz]

Description of the change:

• cmd/operator-sdk/generate/bundle: add NewCmdLegacy that returns a bundle subcommand configured to generate bundles for current project layouts.
• internal/generate/clusterserviceversion: add GenerateLegacy and LegacyOption to generate CSVs for legacy projects.

Motivation for the change: current project layouts should have a generate bundle command to differentiate between bundle and package manifests formats.

/cc @hasbro17 @jmrodri @camilamacedo86

/kind feature

[estroz]

Waiting on #3087, still good to review though.

[camilamacedo86]

Why do we need to create the legacy one? Is not it == the new one and just change the paths?
Is not harder to have the same implementation duplicated instead of having the code centralized to keep it maintained? WDYT? See: #2948 (comment)

PS.; My first approach was to do it as you did here, however, after some comments from @joelanford I think it makes more sense just change the path instead of it as he suggested.

[estroz]

I'm of the opinion that some code duplication is OK, especially with complex command setup like this. It's easier to maintain, then remove when we remove current project layout support.

[hasbro17]

I'm okay with duplication in this case as well. From my experience with trying to reuse the code for generate csv to be used by both the old and new CLI commands it made the underlying command a lot more convoluted.

@camilamacedo86 operator-sdk run local was a much simpler command where we just had to change 1 path.

[hasbro17]

We need a CHANGELOG fragment for adding this cmd to the existing CLI right?

[hasbro17]

We'll need to add a CHANGELOG fragment but LGTM otherwise.