fix(jsii-pacmak): different packages sharing a namespace fail in Bazel (

#4437)

In Bazel, every package is extracted into a separate directory that are all put into the `$PYTHONPATH` individually. However when searching, Python will latch onto the directory that has an `__init__.py` in it and only import from that directory.

Example:

```
eco_package1/
└── site-packages/
    └── eco/
        └── package1/
              └── __init__.py

eco/
└── site-packages/
    └── eco/
        └── __init__.py
```

In this case, the following command will fail:

```py
import eco.package1
```

Because `eco/package1/__init__.py` is not in the same directory as `eco/__init__.py`.

We can fix this by generating a command into every `__init__.py` that uses `pkgutil` to extend the search path for a package; it will put all directories in `$PYTHONPATH` that contain the same package onto the search path for submodules. The command looks like this:

```py
from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)
```

Resolves #3881, thanks to @psalvaggio-dl for providing a repro and doing the research for a fix.

---

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

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/jsii-pacmak/lib/targets/python.ts

@@ -1700,6 +1700,20 @@ class PythonModule implements PythonType {
 
     // Before we write anything else, we need to write out our module headers, this
     // is where we handle stuff like imports, any required initialization, etc.
+
+    // If multiple packages use the same namespace (in Python, a directory) it
+    // depends on how they are laid out on disk if deep imports of multiple packages
+    // will succeed. `pip` merges all packages into the same directory, and deep
+    // imports work automatically. `bazel` puts packages into different directories,
+    // and `import aws_cdk.subpackage` will fail if `aws_cdk/__init__.py` and
+    // `aws_cdk/subpackage/__init__.py` are not in the same directory.
+    //
+    // We can get around this by using `pkgutil` to extend the search path for the
+    // current module (`__path__`) with all packages found on `sys.path`.
+    code.line('from pkgutil import extend_path');
+    code.line('__path__ = extend_path(__path__, __name__)');
+    code.line();
+
     code.line('import abc');
     code.line('import builtins');
     code.line('import datetime');