Extend `vet` so that it produces a warning if any .gitignore files are in the srcs input to a package.

[mads-hartmann]

Is your feature request related to a problem? Please describe

1. It's normal to add build artefacts to .gitignore
2. Leeways' srcs globs makes it easy to include unintended files

Describe the behaviour you'd like

I'd like for vet to provide a warning if any files that are .gitignored are part of the expanded srcs.

Describe alternatives you've considered

Today you can probably manually check for this using a combination of leeway describe, jq and a bit of bash.

Additional context

See gitpod-io/gitpod#14157