First, keep in mind that, in its precise definition, a module is an object in the memory of a Python interpreter, often created by reading one or more files from disk. While we may informally call a disk file such as a/b/c.py a "module," it doesn't actually become one until it's combined with information from several other sources (such as sys.path) to create the module object.
(Note, for example, that two modules with different names can be loaded from the same file, depending on sys.path and other settings. This is exactly what happens with python -m my.module followed by an import my.module in the interpreter; there will be two module objects, __main__ and my.module, both created from the same file on disk, my/module.py.)
A package is a module that may have submodules (including subpackages). Not all modules can do this. As an example, create a small module hierarchy:
$ mkdir -p a/b
$ touch a/b/c.py
Ensure that there are no other files under a. Start a Python 3.4 or later interpreter (e.g., with python3 -i) and examine the results of the following statements:
import a
a ? <module 'a' (namespace)>
a.b ? AttributeError: module 'a' has no attribute 'b'
import a.b.c
a.b ? <module 'a.b' (namespace)>
a.b.c ? <module 'a.b.c' from '/home/cjs/a/b/c.py'>
Modules a and a.b are packages (in fact, a certain kind of package called a "namespace package," though we wont' worry about that here). However, module a.b.c is not a package. We can demonstrate this by adding another file, a/b.py to the directory structure above and starting a fresh interpreter:
import a.b.c
? ImportError: No module named 'a.b.c'; 'a.b' is not a package
import a.b
a ? <module 'a' (namespace)>
a.__path__ ? _NamespacePath(['/.../a'])
a.b ? <module 'a.b' from '/home/cjs/tmp/a/b.py'>
a.b.__path__ ? AttributeError: 'module' object has no attribute '__path__'
Python ensures that all parent modules are loaded before a child module is loaded. Above it finds that a/ is a directory, and so creates a namespace package a, and that a/b.py is a Python source file which it loads and uses to create a (non-package) module a.b. At this point you cannot have a module a.b.c because a.b is not a package, and thus cannot have submodules.
You can also see here that the package module a has a __path__ attribute (packages must have this) but the non-package module a.b does not.