find_imports: only list missing packages

[e-nikolov]

Currently pysandbox tries to micropip.install() all packages from imports even if they are already loaded, e.g.:

• packages from the standard library like "os"
• packages loaded previously from another call to micropip.install()
• relative imports from the filesystem

It works, but there are many errors in the dev console.

This PR changes find_imports() to filter out the existing packages to avoid the errors.

[bugzpodder]

Thanks. do you have a preference between pkgutil.iter_modules vs importlib.util.find_spec. I was using the former one.

[e-nikolov]

https://docs.python.org/3/library/pkgutil.html mentions that some of the functions in pkgutil were deprecated in Python 3.12 and will be removed in Python 3.14 and to use importlib.util.find_spec(). Not sure about pkgutil.iter_modules specifically.