fix(jsii): some submodules are not exported from aws-cdk-lib

[rix0rrr]

This bug is due to a crazy interaction between the publication
module, the __all__ array, and imports we might generate to do
type checking in a package that uses submodules.

Context

Here's what you need to know:

• __all__ is normally used to define the symbols and submodules
  that should be imported when a consumer does from mymod import *.
• publication is used to hide non-public symbols from library
  consumers. It will move all symbols that are NOT in __all__ to
  a _private submodule at runtime (it's overloading __all__ a
  little here, but this seems generally okay).
• We generate imports at the top of the file for the modules that are
  referenced in a module, so that we can use them in type annotations.

Example

This is what the currently generated code for the aws_cdk module
(of CDK's v2 library) currently (roughly) looks like:

import publication
from .cloud_assembly_schema import AmiContextQuery as _AmiContextQuery_74bf4b1b

__all__ = ['Annotations', 'App', ..., 'ValidationResults']

publication.publish()

from . import aws_s3
from . import cloud_assembly_schema
...

Given this setup, the following happens:

$ python3
Python 3.9.5 (default, Jun 27 2021, 10:29:41)
>>> import aws_cdk
>>> print(aws_cdk.aws_s3)
<module 'aws_cdk.aws_s3' from '.../aws_cdk/aws_s3/__init__.py'>
>>> print(aws_cdk.cloud_assembly_schema)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'aws_cdk' has no attribute 'cloud_assembly_schema'

Explanation

Why does this happen?

• __all__ does not contain any submodules
• When publication.publish() is called, all symbols in the calling
  namespace that are missing from __all__ are hidden.
• Crucially, this does include cloud_assembly_schema (because we
  imported that submodule to import some symbols for type checking
  purposes) but it doesn't include modules that haven't been imported
  yet.
• Most modules get imported after publication.publish() is called,
  and so are not subject to hiding anymore. This is why we never noticed
  this problem before.
• Every module only gets imported once, so even though we generate a
  second from . import cloud_assembly_schema line underneath
  publication.publish(), at that point the line doesn't do anything
  anymore.

Solution

Potential solutions are:

• Add submodules to __all__.
• Remove the call to publication.publish().
• Make a variant of publication that doesn't hide submodules.
• Don't generate actual imports for the type annotations.

This PR picks the first solution, as it seems simplest. The only
downside would have been that import * would then start importing all
of the CDKv2 library, but that's happening already anyway because of all
the imports we generate ourselves.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[MrArnoldPalmer]

Looks straightforward 👍