windows: support building shared library distributions

Up until this point we produced a special Python distribution that
was statically linked. This can be useful in some scenarios. But
it can be very limiting because Python extensions need to be
built specifically against this distribution. This rules out
reusing existing binary extensions published on PyPI and
externalizes a lot of hardship on downstream consumers.

This commit teaches our build system to produce a more traditional
Python distribution. We have a pythonXY.dll for the core library
and .pyd files for extensions. There are also .dlls for various
dependencies instead of static libraries.

Closes #22.

Author: indygreg

README.rst

@@ -277,7 +277,7 @@ without having to resort to heuristics.
 The file contains a JSON map. This map has the following keys:
 
 version
-   Version number of the file format. Currently ``3``.
+   Version number of the file format. Currently ``4``.
 
 os
    Target operating system for the distribution. e.g. ``linux``, ``macos``,
@@ -304,6 +304,18 @@ python_stdlib
    Relative path to Python's standard library (where ``.py`` and resource
    files are located).
 
+link_mode
+   How `libpython` is linked. Values can be one of the following:
+
+   `static`
+      Statically linked.
+
+   `shared`
+      Dynamically linked. (A `libpythonXY` shared library will be part
+      of the distribution.)
+
+   (Version 4 or above only.)
+
 build_info
    A map describing build configuration and artifacts for this distribution.
 
@@ -340,7 +352,7 @@ custom set of built-in extension modules.
 This map has the following keys:
 
 core
-   A map describing the core Python distribution (essentially libpython).
+   A map describing the core Python distribution (essentially `libpython`).
 
    objs
       An array of paths to object files constituting the Python core distribution.
@@ -351,6 +363,14 @@ core
    links
       An array of linking requirement maps. (See below for data format.)
 
+   shared_lib
+      Path to a shared library representing `libpython`. May not be defined.
+      (Version 4 or above only.)
+
+   static_lib
+      Path to a static library representing `libpython`. May not be defined.
+      (Version 4 or above only.)
+
 extensions
    A map of extension names to an array of maps describing candidate extensions.
 
@@ -425,6 +445,10 @@ extensions
       Boolean indicating if this extension is required to initialize the Python
       interpreter.
 
+   shared_lib
+      The path to a shared library defining this extension module. May not
+      be defined. (Version 4 or above only.)
+
    static_lib
       The path to a static library defining this extension module. May not
       be defined.

cpython-unix/build.py

@@ -649,6 +649,7 @@ def build_cpython(
             "python_exe": "install/bin/%s" % fully_qualified_name,
             "python_include": "install/include/%s" % fully_qualified_name,
             "python_stdlib": "install/lib/python%s" % entry["version"][0:3],
+            "link_mode": "static",
             "build_info": python_build_info(
                 build_env,
                 platform,