Windows: Make python3.dll optional to include with standalone_dynamic windows distribution

**UPDATED** 1/21/2021 to reflect my new understanding that `python3.dll` is linked to by some extension modules to access [Python's stable ABI](https://docs.python.org/3/c-api/stable.html).

The default `standalone_dynamic` flavor of Python distribution on Windows with MSVC does not link with `python3.dll`. To see this, create a minimal standalone-dynamic executable:

```bzl
def make():
dist = default_python_distribution(flavor="standalone_dynamic")
files = FileManifest()
files.add_python_resource(
".",
dist.to_python_executable(
name="pyapp",
packaging_policy=dist.make_python_packaging_policy(),
config=dist.make_python_interpreter_config(),
))
return files

register_target("install", make, default=True)
resolve_targets()
```
After runnning `pyoxidizer build`, `\build\x86_64-pc-windows-msvc\debug\install` now contains `lib`, `pyapp.exe`, `python38.dll`, and `python3.dll`. Now we can use [`dumpbin`](https://docs.microsoft.com/en-us/cpp/build/reference/dumpbin-reference) with [`/dependents`](https://docs.microsoft.com/en-us/cpp/build/reference/dependents) to verify that `python3.dll` is not actually used. Change into that directory and run

```console
> dumpbin pyapp.exe /dependents
Microsoft (R) COFF/PE Dumper Version 14.26.28806.0
Copyright (C) Microsoft Corporation. All rights reserved.

Dump of file pyapp.exe

File Type: EXECUTABLE IMAGE

Image has the following dependencies:

ADVAPI32.dll
KERNEL32.dll
python38.dll
VCRUNTIME140.dll
api-ms-win-crt-string-l1-1-0.dll
api-ms-win-crt-heap-l1-1-0.dll
api-ms-win-crt-utility-l1-1-0.dll
api-ms-win-crt-convert-l1-1-0.dll
api-ms-win-crt-runtime-l1-1-0.dll
api-ms-win-crt-math-l1-1-0.dll
api-ms-win-crt-stdio-l1-1-0.dll
api-ms-win-crt-locale-l1-1-0.dll

Summary

3000 .data
47000 .pdata
1ABD000 .rdata
2000 .reloc
1000 .rsrc
344000 .text
```
Repeat the above experiment replacing `pyapp.exe` with `python38.dll` and the wildcard `lib\*.pyd`. None of them load `python3.dll`.

Because `python3.dll` is not needed by default, it would be nice to have an option removing it and some documentation pointing at [PEP 384](https://www.python.org/dev/peps/pep-0384/#linkage), which explains that `python3.dll` is for the stable ABI. This would make it easier to de-clutter an installation, and it hints to users of PyOxidizer how to start dealing with DLL availability/compatibility issues.

(Cf. #353 requesting that PyOxidizer place `vcruntime140.dll` next to the `.exe`.)

Start with the standalone_dynamic Windows distribution example and use dumpbin /dependents on pyapp.exe, python38.dll, and lib\*.pyd to confirm current dependencies. Read the packaging and distribution behavior around python3.dll and PEP 384's stable-ABI linkage. Done means python3.dll can be omitted through an explicit option and the relevant documentation explains when it is needed.

Written by the indexing model from the issue text.