Fixes#2876 with @paulmelnikow's suggestion
Moved imports of `ServiceTester` and `createServiceTester` to a separate file so that dev dependencies are not imported by service classes.
Continue to implement #2698:
- Add `core/base-service/index.js` (but hold off on moving the things it imports)
- Add shortcuts in `services/index.js` for Base*Service, errors, and deprecatedService. This file will be streamlined later to avoid cluttering it with rarely used bits.
- Apply consistent ordering of imports and use of `module.exports` in testers.
- Remove some renaming of imports.
- Remove obsolete tests here and there.
1. Add validation to BaseSvgScrapingService and update readthedocs accordingly.
2. Rewrite vso and add more tests. Rename it internally to azure-devops. URLs are still `/vso` for now. Should we make a way to let a service register multiple URL patterns?
3. Handle shared code using a functional pattern instead of inheritance. This comes from a discussion https://github.com/badges/shields/pull/2031#issuecomment-417893819. I like the functional approach because it's more direct, nimble, and easy to reason about; plus it allows services to grow from a family of one to two more easily.
