Previously we could rely on knowing that packaged buildpacks would be written to a location within the `target` directory configured in Crate. This was easy to determine through program code and could be supplied to `find_buildpack_dirs` so that packaged buildpacks weren't accidentally included when searching for buildpacks.

With the changes introduced in [#583](#583) it is no longer possible to easily determine the output location for packaged buildpacks. This PR adds the [ignore](https://crates.io/crates/ignore) crate to provide directory iteration that respects standard ignore files. This will give the user the ability to configure where we search within a project for buildpacks.

* updated error message
* using iterator instead of loop for finding buildpack dirs logic
* added a quick test to ensure that ignores work
* updated changelog entry

* updated error message
* using iterator instead of loop for finding buildpack dirs logic
* added a quick test to ensure that ignores work
* updated changelog entry

* updated test comments
* added PR link to changelog entry