Fix finding typeshed user install on Windows (#3991)

Python 3.5+ installs to `%APPDATA%\python\PythonXY`. Mypy, when installed
as user, installs typeshed to `%APPDATA%\python\lib\mypy`. This fixes the
issues with resolving the path to typeshed. Additionally, more debug
information is added to the error message to assist in debugging issues
like this in the future.

Fixes #3988

Author: emmatyping

mypy/build.py

@@ -223,10 +223,13 @@ def default_data_dir(bin_dir: Optional[str]) -> str:
             # or .../blah/lib64/python3.N/dist-packages/mypy/build.py (Gentoo)
             # or .../blah/lib/site-packages/mypy/build.py (Windows)
             # blah may be a virtualenv or /usr/local.  We want .../blah/lib/mypy.
+            # On Windows, if the install is .../python/PythonXY/site-packages, we want
+            # .../python/lib/mypy
             lib = parent
             for i in range(2):
                 lib = os.path.dirname(lib)
-                if os.path.basename(lib) in ('lib', 'lib32', 'lib64'):
+                if os.path.basename(lib) in ('lib', 'lib32', 'lib64') \
+                        or os.path.basename(lib).startswith('python'):
                     return os.path.join(os.path.dirname(lib), 'lib/mypy')
         subdir = os.path.join(parent, 'lib', 'mypy')
         if os.path.isdir(subdir):
@@ -299,9 +302,10 @@ def default_lib_path(data_dir: str,
     if sys.platform != 'win32':
         path.append('/usr/local/lib/mypy')
     if not path:
-        print("Could not resolve typeshed subdirectories. If you are using MyPy"
-              "from source, you need to run \"git submodule --init update\"."
-              "Otherwise your MyPy install is broken.", file=sys.stderr)
+        print("Could not resolve typeshed subdirectories. If you are using mypy\n"
+              "from source, you need to run \"git submodule --init update\".\n"
+              "Otherwise your mypy install is broken.\nPython executable is located at "
+              "{0}.\nMypy located at {1}".format(sys.executable, data_dir), file=sys.stderr)
         sys.exit(1)
     return path