[Binary provisioning] Move Launcher to internal/cmd package

[codebien]

What?

It merges the launcher package into internal/cmd.

Why?

At the moment, k6 isn't designed to have a separate package for startup ops, and doing so was causing problems. So, as we want to avoid major refactoring, then it necessitates adhering to this structure.
This is especially important for the config, which is internal to the 'cmd' package. While exporting a dedicated 'config' package would be beneficial, it would require significant time and effort. For now, maintaining the current structure will be simple, and simplicity is key here.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

[joanlopez]

Looks good! 👍🏻

Arguably, this is yet another small reason to use something along the lines of "executor" for #4708, as it is mainly called from the public function Execute() and the execute.go file.