buildEnv: build the whole tree of directories to pathsToLink

This patch fixes #16614 and #16741.

The first issue was caused by the fact that both `/share` and
`/share/fish/vendor_completions.d` end in the `pathsToLink`. The
`pkgs/build-support/buildenv/builder.pl` creates `/share`, then links
`/share/fish` under `/share` and then tries to create the directory
`/share/fish/vendor_completions.d` and fails because it already exists.

The simplest way to reproduce the issue is to build the next Nix
expression:

```nix
let pkgs = import <nixpkgs> { };
in pkgs.buildEnv {
name = "buildenv-issue";

paths = [
pkgs.fish
pkgs.vim
];

pathsToLink = [
"/share"
"/share/fish/vendor_completions.d"
];
}
```

The second issue is more critical and was caused by the fact findFiles
doesn't recurse deep enough. It stops at first unique directory for the
package (e.g., "/share" or even "/") and later the scripts decides it
shouldn't link it as it doesn't match pathsToLink (e.g., "/share/fish"),
so the result is empty.

The test:
```nix
let pkgs = import <nixpkgs> { };
in pkgs.buildEnv {
name = "buildenv-issue";

paths = [
pkgs.fish
pkgs.vim
];

pathsToLink = [
"/share/fish/functions"
];
}
```

or

```nix
let pkgs = import <nixpkgs> { };
in pkgs.buildEnv {
name = "buildenv-issue";

paths = [
pkgs.vim
];

pathsToLink = [
"/share"
];
}
```

pkgs/build-support/buildenv/builder.pl

···
 
 my %symlinks;
 
+# Add all pathsToLink and all parent directories.
+#
+# For "/a/b/c" that will include
+# [ "", "/a", "/a/b", "/a/b/c" ]
+#
+# That ensures the whole directory tree needed by pathsToLink is
+# created as directories and not symlinks.
+$symlinks{""} = ["", 0];
 for my $p (@pathsToLink) {
-    $p = "" if $p eq "/";
-    $symlinks{$p} = ["", 0];
+    my @parts = split '/', $p;
+
+    my $cur = "";
+    for my $x (@parts) {
+        $cur = $cur . "/$x";
+        $cur = "" if $cur eq "/";
+        $symlinks{$cur} = ["", 0];
+    }
 }
 
 sub findFiles;