Moves datasource, manager, platform and versioning code from lib/ into new lib/modules/

BREAKING CHANGE: External tools must update paths to datasource, manager, platform and versioning

* refactor: introduce Fixtures util class

* refactor: align yarn.spec

* refactor: move fs mocks to __mocks__ folder

* refactor: improve fs-extra mock

* refactor: move callsite to dev deps

* refactor: use fixtures class

* refactor: use fixtures class

* refactor: reduce external variables in tests

* refactor: improve Fixtures

* refactor: add type for realFs

* refactor: remove obsolete snapshot

* fix: by comments

* refactor: provide ability to spy on mocked fs methods

* refactor: fix build

* refactor: fix by comments

* refactor: add docs

* refactor: fix unit test

* refactor: return called times checks

* refactor: fix by comments

* refactor: adjust unit test

* refactor: adjust unit test

* refactor: fix unit test

* refactor: fix by comments

* refactor: fix by comments

* refactor: update jsdoc

* refactor: fix by comments

Co-authored-by: Rhys Arkins <rhys@arkins.net>
Co-authored-by: Jamie Magee <jamie.magee@gmail.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

- Refactor: move fixtures to corresponding directories
- Delete unused fixtures

Resolves: #3328 

Wraps the `got` library in our own wrapper to add:
- Renovate user agent
- Memory cache wrapper