Fix duplicated imports with importlib mode and doctest-modules (#11148)

The initial implementation (in #7246) introduced the `importlib` mode, which
never added the imported module to `sys.modules`, so it included a test
to ensure calling `import_path` twice would yield different modules.

Not adding modules to `sys.modules` proved problematic, so we began to add the imported module to `sys.modules`
in #7870, but failed to realize that given we are now changing `sys.modules`, we might
as well avoid importing it more than once.

Then #10088 came along, passing `importlib` also when importing application modules
(as opposed to only test modules before), which caused problems due to imports
having side-effects and the expectation being that they are imported only once.

With this PR, `import_path` returns the module immediately if already in
`sys.modules`.

Fix #10811
Fix #10341

src/_pytest/pathlib.py

@@ -523,6 +523,8 @@ def import_path(
 
     if mode is ImportMode.importlib:
         module_name = module_name_from_path(path, root)
+        with contextlib.suppress(KeyError):
+            return sys.modules[module_name]
 
         for meta_importer in sys.meta_path:
             spec = meta_importer.find_spec(module_name, [str(path.parent)])