dry-run: Dependabot::ConfigFile

[thepwagner]

This introduces the Dependabot::Config module, starring the Dependabot::Config::File class.

This PR wraps the configuration stored in dependabot.yaml, with helpers to locate the configuration relevant to the active ecosystem/directory.

This is incomplete, only extracting the features to provide:

• use the commit-message options, if --pull-request is enabled
• use the ignore: conditions, if IGNORE_CONDITIONS is not set (following dry-run: ignore conditions from env #3507)

[feelepxyz]

Nice! I also started on some of these primitives a while ago and wondering if we want to crib some stuff from there?

I tried to implement the config file fetcher as another file fetcher: https://github.com/dependabot/dependabot-core/compare/feelepxyz/cli-experiment?expand=1#diff-7f068f9118efc60b552a73767f52e71a60020c0690b239c292b4fc050d421782R8-R42

I also copied over the whole schema and a lot of the parsing logic which we probably don't want to get this over the line.

[thepwagner]

I tried to implement the config file fetcher as another file fetcher

SGTM, I'll make this change.

It's definitely a chicken/egg problem: the FileFetcher instance is informed by the package_manager, but the active package_manager s are contained the config file.

copied over the whole schema and a lot of the parsing logic which we probably don't want to get this over the line

I wasn't keen to bring "real" validation (e.g. the JSON schema), it seems harmless scope creep but would have the maintenance cost of requiring the schema be maintained in two places.
There's some demand, e.g. #1957 #1927 - I just didn't want to go there (yet).

[feelepxyz]

LGTM overall, left a few minor comments, feel free to merge when you've folded in what makes sense 👍

[feelepxyz]

🙌