feat: reimplement findFilePaths using trees (#950)

* refactor: replace github code search with getTree/manual search (#944)

Previously Renovate was using the GitHub code search API once for each filename we are looking for. Instead, we now retrieve a list of files in the repository and cache it so it can be reused for filename matches.

Closes #939

* fix: use recursive=true

* fix: reset fileList for every repo (github)

* feat: implement findFilePaths on gitlab (#945)

* refactor: do not check manually for root package.json

* fix: reset fileList for every repo (gitlab)

* fix tests