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/Makefile.pre.in b/Makefile.pre.in
index 29d7e34..3e4b20b 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -304,7 +304,7 @@
 
 
 PEGEN_HEADERS= \
-		$(srcdir)/Include/pegen_interface.h \
+		$(srcdir)/Include/internal/pegen_interface.h \
 		$(srcdir)/Parser/pegen/pegen.h \
 		$(srcdir)/Parser/pegen/parse_string.h