Disallow Submodules as main-module in rio.toml

[mad-moo]

It is currently possible to specify submodules as the app's main module. For example, the rio.App can be located at foo.bar instead of foo. While this is convenient and seems to sometimes work, it is not actually supported.

Rio relies on being able to determine the location of the file, by looking for a file/folder with that name in the project directory. Submodules don't adhere to this though - the Python code can run arbitrary operations, which makes it impossible to locate the correct files in general.

If we want to support this, all code should be reviewed and possibly some restrictions defined & documented. Until then, Rio should explicitly check to make sure the specified module is not a submodule and notify the user.