community: Standardise tool import for arxiv & semantic scholar (#23578)

- **Description:** Fixing the way users have to import Arxiv and
Semantic Scholar
- **Issue:** Changed to use `from langchain_community.tools.arxiv import
ArxivQueryRun` instead of `from langchain_community.tools.arxiv.tool
import ArxivQueryRun`
    - **Dependencies:** None
    - **Twitter handle:** Nope

libs/community/langchain_community/tools/arxiv/__init__.py

@@ -1 +1,6 @@
+from langchain_community.tools.arxiv.tool import ArxivQueryRun
+
 """Arxiv API toolkit."""
+"""Tool for the Arxiv Search API."""
+
+__all__ = ["ArxivQueryRun"]

libs/community/langchain_community/tools/semanticscholar/__init__.py

@@ -1 +1,6 @@
+from langchain_community.tools.semanticscholar.tool import SemanticScholarQueryRun
+
 """Semantic Scholar API toolkit."""
+"""Tool for the Semantic Scholar Search API."""
+
+__all__ = ["SemanticScholarQueryRun"]