text: Title: Fix strict typecheck for cirq importers
  Body: **Description of the issue**
  
  Clients importing from cirq fail strict type checking as follows:
  
  ```
  $ mypy --strict -c "from cirq import Circuit"
  <string>:1: error: Module "cirq" does not explicitly export attribute "Circuit"  [attr-defined]
  ```
  
  This is because our typecheck setup in [mypy.ini](https://github.com/quantumlib/Cirq/blob/f56a799c4dff188df9cb91c2de524a7953938e48/dev_tools/conf/mypy.ini#L1-L6) does not enforce [no_implicit_reexport](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport).
  
  **Solution**
  
  1. add `no_implicit_reexport = true` to mypy.ini
  2. update `__init__.py` files to reexport symbols, ie, replace `from foo import bar` with `from foo import bar as bar`
  
  **Cirq version**
  
  1.4.1 at f56a799c4dff188df9cb91c2de524a7953938e48