py/builtinimport: Allow builtin modules to be packages.

To use this:
 - Create a built-in module, and add the module object as a member of the
   parent module's globals dict.
 - The submodule can set its `__name__` to either `QSTR_foo_dot_bar` or
   `QSTR_bar`. The former requires using qstrdefs(port).h to make the qstr.

Because `bar` is a member of `foo`'s globals, it is possible to write
`import foo` and then immediately use `foo.bar` without importing it
explicitly. This means that if `bar` has an `__init__`, it will not be
called in this situation, and for that reason, sub-modules should not have
`__init__` methods. If this is required, then all initalisation for
sub-modules should be done by the top-level module's (i.e. `foo`'s)
`__init__` method.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

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