convert last small purpose of builtin.pm to C and NOOP require's I/O

-builtin.pm is now primarily for POD and .pm indexing tools, core, CPAN or
user written.  It also is a backup mechanism for very strange %INC
localization, clearing, or manipulation done by users, probably in a .t,
and whatever %INC manipulation is being done is probably developer error.
-This removes all the libc/kernel I/O calls for builtin.pm, and Perl code
parser overhead.
-A large benefit is, this commit is 50% of the work, to make

perl -E 'say "hi";'

"/lib"-less or not dependent on any file I/O. perl.bin, libperl.so,
and miniperl.bin should be able to execute as a standalone binary.
If perl -e "1;" doesn't need a dozen separate library files,
perl -E "1;" also shouldn't need a dozen files.

perl -E "say 'Hello world';" should work, even with a broken perl
installation or unreachable "/lib/*.pm"s or broken "portable" perls.

Only a feature.pm dep is left, for -E to be lib-less.  That is for another
patch and PR in the the future.

builtin.c

@@ -774,6 +774,9 @@ XS(XS_builtin_import)
 void
 Perl_boot_core_builtin(pTHX)
 {
+    HV * inc_hv;
+    GV * ver_gv;
+    SV * ver_sv;
     I32 i;
     for(i = 0; builtins[i].name; i++) {
         const struct BuiltinFuncDescriptor *builtin = &builtins[i];
@@ -807,6 +810,13 @@ Perl_boot_core_builtin(pTHX)
     }
 
     newXS_flags("builtin::import", &XS_builtin_import, __FILE__, NULL, 0);
+
+    inc_hv = GvHVn(PL_incgv);
+    hv_store(inc_hv, "builtin.pm", STRLENs("builtin.pm"), newSVpvs(__FILE__), 0);
+    ver_gv = gv_fetchpvs("builtin::VERSION", GV_ADDMULTI, SVt_PV);
+    ver_sv = GvSV(ver_gv);
+    /* Remember to keep $VERSION in this file and $VERSION in builtin.pm synced. */
+    sv_setpvs(ver_sv, "0.016");
 }
 
 /*

lib/builtin.pm

@@ -1,10 +1,13 @@
-package builtin 0.015;
+package builtin 0.016;
 
 use v5.40;
 
 # All code, including &import, is implemented by always-present
-# functions in the perl interpreter itself.
-# See also `builtin.c` in perl source
+# functions in the perl interpreter itself in `builtin.c`.
+#
+# $builtin::VERSION and %INC are also set by the interpreter in `builtin.c`
+# since this file is a NOOP.  Therefore this file is unlikely to ever execute
+# and primarily serves as POD.
 
 __END__
 

lib/builtin.t

@@ -645,6 +645,13 @@ EOS
     }
 }
 
+like($INC{'builtin.pm'}, qr/builtin\.c/, 'check that \'builtin.pm\' is in %INC');
+{
+    my $xsv = $builtin::VERSION;
+    delete $INC{'builtin.pm'};
+    eval "use builtin;";
+    is($xsv, $builtin::VERSION, 'XS $VERSION matches PP $VERSION');
+}
 # vim: tabstop=4 shiftwidth=4 expandtab autoindent softtabstop=4
 
 done_testing();

pod/perldelta.pod

@@ -123,11 +123,22 @@ XXX Remove this section if F<Porting/corelist-perldelta.pl> did not add any cont
 
 =over 4
 
-=item *
-
-L<XXX> has been upgraded from version A.xx to B.yy.
-
-XXX If there was something important to note about this change, include that here.
+=item builtin.pm
+
+L<builtin.pm|/builtin.pm> has been upgraded from version 0.015 to 0.016.
+As an optimization the C<builtin::> package and module is now fully
+implemented in C to save on file system I/O and startup time.
+
+Specifically the C<use builtin;> statement  and any permutations of
+C<use builtin;> will not internally perform a per process first time
+C<require;> anymore and C<$INC{'builtin.pm'}> is already filled in at
+process startup.  All C<.pm> functionality is embedded in the interpreter at
+startup now.  This optimization removes all I/O calls and time overhead of
+parsing the C<builtin.pm> file.  The C<builtin.pm> file will remain as
+documentation.
+
+C<builtin::>'s C<import()>, C<$VERSION>, and all available subroutines/APIs
+have not changed.  No changes are required to previously written code.
 
 =back