Fix missing importlib.util import in exact_search.py

[peylix]

Description

This PR fixes an AttributeError: module 'importlib' has no attribute 'util' that occurs when importing DenseRetrievalExactSearch.

Problem

Previously when using DenseRetrievalExactSearch, I encountered the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "beir/retrieval/search/dense/__init__.py", line 3, in <module>
    from .exact_search import DenseRetrievalExactSearch
  File "beir/retrieval/search/dense/exact_search.py", line 8, in <module>
    if importlib.util.find_spec("faiss") is not None:
       ^^^^^^^^^^^^^^
AttributeError: module 'importlib' has no attribute 'util'

After investigation, I found the file beir/retrieval/search/dense/exact_search.py currently imports only the importlib module but attempts to use importlib.util.find_spec(). In Python, importing a parent module does not automatically import its submodules, leading to an AttributeError.

Solution

Explicitly import the importlib.util submodule:

import importlib.util

This is consistent with other files in the codebase (e.g., faiss_search.py, faiss_index.py) that already use the correct import.

Test Environment

• Python: 3.12
• BEIR: 2.2.0+
• OS: macOS