Enable bundling executors with commands

There should be an option for distributing commands bundled together with some executors (builders, deployers, etc).

- Commands should load all `*.yaml` files from `/executors/` directory.
- Commands should prevent overriding bundled executors. When project defines executor with the same name as a bundled one, command should exit with an error.

Start by tracing how commands currently discover and load project executors, then inspect how the command binary's parent directory can be identified. Verify the YAML loading path and error handling for duplicate executor names; done means bundled *.yaml executors are loaded and a name collision causes the command to exit with an error.

Written by the indexing model from the issue text.