The initial implementation (in pytest-dev#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 pytest-dev#7870, but failed to realize that given we are now changing `sys.modules`, we might
as well avoid importing it more than once.

Then pytest-dev#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 pytest-dev#10811, pytest-dev#10341