bpo-35134: Add Include/cpython/import.h header file (GH-14213)

* Add Include/cpython/import.h and Include/internal/pycore_import.h
  header files.
* Move _PyImport_ReInitLock() to the internal C API. Don't export the
  symbol anymore.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 09a63c0..4fd2607 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -129,6 +129,7 @@
     <ClInclude Include="..\Include\cpython\abstract.h" />
     <ClInclude Include="..\Include\cpython\dictobject.h" />
     <ClInclude Include="..\Include\cpython\fileobject.h" />
+    <ClInclude Include="..\Include\cpython\import.h" />
     <ClInclude Include="..\Include\cpython\initconfig.h" />
     <ClInclude Include="..\Include\cpython\object.h" />
     <ClInclude Include="..\Include\cpython\objimpl.h" />
@@ -158,14 +159,15 @@
     <ClInclude Include="..\Include\import.h" />
     <ClInclude Include="..\Include\internal\pycore_accu.h" />
     <ClInclude Include="..\Include\internal\pycore_atomic.h" />
-    <ClInclude Include="..\Include\internal\pycore_code.h" />
     <ClInclude Include="..\Include\internal\pycore_ceval.h" />
+    <ClInclude Include="..\Include\internal\pycore_code.h" />
     <ClInclude Include="..\Include\internal\pycore_condvar.h" />
     <ClInclude Include="..\Include\internal\pycore_context.h" />
     <ClInclude Include="..\Include\internal\pycore_fileutils.h" />
     <ClInclude Include="..\Include\internal\pycore_getopt.h" />
     <ClInclude Include="..\Include\internal\pycore_gil.h" />
     <ClInclude Include="..\Include\internal\pycore_hamt.h" />
+    <ClInclude Include="..\Include\internal\pycore_import.h" />
     <ClInclude Include="..\Include\internal\pycore_initconfig.h" />
     <ClInclude Include="..\Include\internal\pycore_object.h" />
     <ClInclude Include="..\Include\internal\pycore_pathconfig.h" />