Closes #1888

Fix pathRules migration

This PR adds basic support for requirements.txt. Currently it works on fully specified (pinned) versions only, so is disabled by default. Enable it by setting `pip_requirements.enabled = true` in config.

pathRules are now migrated to packageRules.

Closes #1880 

This PR adds initial support for buildkite plugin renovation.

It supports `plugin-name` or `my/plugin-name` plugins, and fully specified semver versions only (e.g. `v1.3.2`). Currently it will always propose an upgrade to the latest version available, e.g. if current version is v1.3.1 and both v1.3.2 and v2.0.0 exist then v2.0.0 will be proposed. Looks for any yml file in the `.buildkite/` directory.

Closes #1869 

The matchCurrentVersion option sets a range of versions that a package update can be in. If the package's current version doesn't satisfy the matchCurrentVersion range, it won't match the rule.

Closes #1771

Closes #1871 

This PR adds the capability so that the onboarding commit message (“Add renovate.json”) will have a prefix if semanticCommits is explicitly set to true by the bot administrator.

e.g. “renovate a/b —semantic-commits=true —semantic-commit-type=foo” will result in the onboarding commit being “foobar: add renovate.json”.

Closes #1867