Fix building Pipeline with YAML

[tanaysoni]

With #785, a feature to create a Pipeline from a YAML configuration file was introduced. It iteratively loads components as defined in the YAML.

To support the loading of the components, the BaseComponent class registers all subclasses using the __init_subclass__(). However, it may happen that a subclass is not yet evaluated by a Python interpreter, leading to a "component not found" error. The error did not get surfaced in the test cases as all the components used in the tests had already been imported and thus registered as subclasses of the BaseComponent.

This PR adds explicit import of components in their respective modules' __init__.py files.

Optionally, as a side benefit, it could simplify the import statements. For instance,

from haystack.document_store.elasticsearch import ElasticsearchDocumentStore

can be replaced with:

from haystack.document_store import ElasticsearchDocumentStore

[tholor]

LGTM