[Fixtures] Externalized fixtures

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | none

This enables the use of SyliusCoreBundle without the default fixtures.

Without this, it is impossible to override the default fixtures with your own fixtures. Requires a new sub-tree-split.