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.
- Replace the idea of color schemes with the idea of named colors (since none of our colorschemes have used `colorA`)
- Pass through the normalized color to `_shields_test` to harmonize with BaseService and simplify testing
- Update service tests
- Move responsibility for color generation into the npm package
- Remove several color helper functions and their tests
- Update gh-badge public API to accept `color` and `labelColor`
This is a precursor to refactoring some of the logo code for #2473.
