refact!: Remove symbols under the haystack.document_stores namespace

[masci]

Related Issues

No issue, preparation work for plugging integrations import paths under the haystack. namespace.

Proposed Changes:

Remove symbols under haystack.document_stores and haystack.component.retrievers, now symbols must be imported explicitly, e.g.

from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack.components.retrievers.in_memory import InMemoryBM25Retriever

instead of

from haystack.document_stores import InMemoryDocumentStore
from haystack.components.retrievers import InMemoryEmbeddingRetriever

How did you test it?

Unit tests

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 7478258034

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 87.074%

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Totals
Change from base Build 7464280343:	0.2%
Covered Lines:	4217
Relevant Lines:	4843

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💛 - Coveralls

[silvanocerza]

🚀