bpo-40334: Improve various PEG-Parser related stuff (GH-19669)

The changes in this commit are all related to @vstinner's original review comments of the initial PEP 617 implementation PR.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index d795c4d..3484f44 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -161,6 +161,7 @@
     <ClInclude Include="..\Include\graminit.h" />
     <ClInclude Include="..\Include\grammar.h" />
     <ClInclude Include="..\Include\import.h" />
+    <ClInclude Include="..\Include\internal\pegen_interface.h" />
     <ClInclude Include="..\Include\internal\pycore_abstract.h" />
     <ClInclude Include="..\Include\internal\pycore_accu.h" />
     <ClInclude Include="..\Include\internal\pycore_atomic.h" />
@@ -213,7 +214,6 @@
     <ClInclude Include="..\Include\parsetok.h" />
     <ClInclude Include="..\Include\patchlevel.h" />
     <ClInclude Include="..\Include\picklebufobject.h" />
-    <ClInclude Include="..\Include\pegen_interface.h" />
     <ClInclude Include="..\Include\pyhash.h" />
     <ClInclude Include="..\Include\pyhash.h" />
     <ClInclude Include="..\Include\py_curses.h" />