Avoid search paths for ImportChecker when possible (#9595)

* Avoid search paths for ImportChecker when possible

If possible it is desirable to look for modules with no context file as
it results in no search paths being given to astroid's find_spec(). This
makes calls to it more uniform and opens up the possibility of effective
caching.

Refs #9310.

Author: crazybolillo

doc/whatsnew/fragments/9310.performance

@@ -0,0 +1,4 @@
+ImportChecker's logic has been modified to avoid context files when possible. This makes it possible
+to cache module searches on astroid and reduce execution times.
+
+Refs #9310.

pylint/checkers/imports.py

@@ -1045,9 +1045,12 @@ def _add_imported_module(self, node: ImportNode, importedmodname: str) -> None:
         base = os.path.splitext(os.path.basename(module_file))[0]
 
         try:
-            importedmodname = astroid.modutils.get_module_part(
-                importedmodname, module_file
-            )
+            if isinstance(node, nodes.ImportFrom) and node.level:
+                importedmodname = astroid.modutils.get_module_part(
+                    importedmodname, module_file
+                )
+            else:
+                importedmodname = astroid.modutils.get_module_part(importedmodname)
         except ImportError:
             pass