gh-131876: extract _hashlib helpers into a separate directory (#136995)

The `Modules/hashlib.h` helper file is now removed and split into multiple files:

* `Modules/_hashlib/hashlib_buffer.[ch]` -- Utilities for getting a buffer view and handling buffer inputs.
* `Modules/_hashlib/hashlib_fetch.h` -- Utilities used when fetching a message digest from a digest-like identifier.
  Currently, this file only contains common error messages as the fetching API is not yet implemented.
* `Modules/_hashlib/hashlib_mutex.h` -- Utilities for managing the lock on cryptographic hash objects.

configure.ac

@@ -7187,6 +7187,7 @@ SRCDIRS="\
   Modules/_decimal \
   Modules/_decimal/libmpdec \
   Modules/_hacl \
+  Modules/_hashlib \
   Modules/_io \
   Modules/_multiprocessing \
   Modules/_sqlite \
@@ -7957,6 +7958,15 @@ PY_STDLIB_MOD_SIMPLE([_codecs_tw])
 PY_STDLIB_MOD_SIMPLE([_multibytecodec])
 PY_STDLIB_MOD_SIMPLE([unicodedata])
 
+###############################################################################
+# Cryptographic primitives
+LIBHASHLIB_INTERNAL_CFLAGS="-I\$(srcdir)/Modules/_hashlib"
+LIBHASHLIB_INTERNAL_LDFLAGS="-lm \$(LIBHASHLIB_INTERNAL_A)"
+LIBHASHLIB_INTERNAL="\$(LIBHASHLIB_INTERNAL_HEADERS) \$(LIBHASHLIB_INTERNAL_A)"
+
+AC_SUBST([LIBHASHLIB_INTERNAL_CFLAGS])
+AC_SUBST([LIBHASHLIB_INTERNAL])
+
 ###############################################################################
 # HACL* compilation and linking configuration (contact: @picnixz)
 #
@@ -8093,7 +8103,9 @@ dnl The EXTNAME is the name of the extension module being built.
 AC_DEFUN([PY_HACL_CREATE_MODULE], [
   AS_VAR_PUSHDEF([v], [[LIBHACL_][$1][_LDFLAGS]])
   AS_VAR_SET([v], [[LIBHACL_][$1][_LIB_${LIBHACL_LDEPS_LIBTYPE}]])
-  PY_STDLIB_MOD([$2], [$3], [], [$LIBHACL_CFLAGS], [\$($v)])
+  PY_STDLIB_MOD([$2], [$3], [],
+    [$LIBHACL_CFLAGS $LIBHASHLIB_INTERNAL_CFLAGS],
+    [\$($v) $LIBHASHLIB_INTERNAL_LDFLAGS])
   AS_VAR_POPDEF([v])
 ])
 
@@ -8174,7 +8186,8 @@ dnl OpenSSL bindings
 PY_STDLIB_MOD([_ssl], [], [test "$ac_cv_working_openssl_ssl" = yes],
   [$OPENSSL_INCLUDES], [$OPENSSL_LDFLAGS $OPENSSL_LDFLAGS_RPATH $OPENSSL_LIBS])
 PY_STDLIB_MOD([_hashlib], [], [test "$ac_cv_working_openssl_hashlib" = yes],
-  [$OPENSSL_INCLUDES], [$OPENSSL_LDFLAGS $OPENSSL_LDFLAGS_RPATH $LIBCRYPTO_LIBS])
+  [$OPENSSL_INCLUDES $LIBHASHLIB_INTERNAL_CFLAGS],
+  [$OPENSSL_LDFLAGS $OPENSSL_LDFLAGS_RPATH $LIBCRYPTO_LIBS $LIBHASHLIB_INTERNAL_LDFLAGS])
 
 dnl test modules
 PY_STDLIB_MOD([_testcapi],