tidy-up: merge root packages directory into projects

To simplify the directory layout.

- OS400 and vms support move from `packages` to `projects`.

- Windows README and `generate.bat` files move from `projects`
  to `projects/Windows`.

Closes #20271

scripts/singleuse.pl

@@ -167,7 +167,7 @@ my %api = (
 
 sub doublecheck {
     my ($f, $used) = @_;
-    open(F, "git grep -Fwle '$f' -- lib ${unittests}packages|");
+    open(F, "git grep -Fwle '$f' -- lib ${unittests}projects|");
     my @also;
     while(<F>) {
         my $e = $_;

scripts/spacecheck.pl

@@ -41,7 +41,8 @@ my @need_crlf = (
 my @double_empty_lines = (
     "RELEASE-NOTES",
     "^lib/.+\\.(c|h)\$",
-    "^packages/",
+    "^projects/OS400",
+    "^projects/vms",
     "^tests/data/test",
     "\\.(m4|py)\$",
 );