bpo-35081: Add pycore_ prefix to internal header files (GH-10263)

* Rename Include/internal/ header files:

  * pyatomic.h -> pycore_atomic.h
  * ceval.h -> pycore_ceval.h
  * condvar.h -> pycore_condvar.h
  * context.h -> pycore_context.h
  * pygetopt.h -> pycore_getopt.h
  * gil.h -> pycore_gil.h
  * hamt.h -> pycore_hamt.h
  * hash.h -> pycore_hash.h
  * mem.h -> pycore_mem.h
  * pystate.h -> pycore_state.h
  * warnings.h -> pycore_warnings.h

* PCbuild project, Makefile.pre.in, Modules/Setup: add the
  Include/internal/ directory to the search paths of header files.
* Update includes. For example, replace #include "internal/mem.h"
  with #include "pycore_mem.h".
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index f65bb5b..ebc35a7 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -112,15 +112,16 @@
     <ClInclude Include="..\Include\graminit.h" />
     <ClInclude Include="..\Include\grammar.h" />
     <ClInclude Include="..\Include\import.h" />
-    <ClInclude Include="..\Include\internal\ceval.h" />
-    <ClInclude Include="..\Include\internal\condvar.h" />
-    <ClInclude Include="..\Include\internal\context.h" />
-    <ClInclude Include="..\Include\internal\gil.h" />
-    <ClInclude Include="..\Include\internal\hamt.h" />
-    <ClInclude Include="..\Include\internal\mem.h" />
-    <ClInclude Include="..\Include\internal\pyatomic.h" />
-    <ClInclude Include="..\Include\internal\pystate.h" />
-    <ClInclude Include="..\Include\internal\warnings.h" />
+    <ClInclude Include="..\Include\internal\pycore_atomic.h" />
+    <ClInclude Include="..\Include\internal\pycore_ceval.h" />
+    <ClInclude Include="..\Include\internal\pycore_condvar.h" />
+    <ClInclude Include="..\Include\internal\pycore_context.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_mem.h" />
+    <ClInclude Include="..\Include\internal\pycore_state.h" />
+    <ClInclude Include="..\Include\internal\pycore_warnings.h" />
     <ClInclude Include="..\Include\intrcheck.h" />
     <ClInclude Include="..\Include\iterobject.h" />
     <ClInclude Include="..\Include\listobject.h" />