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"
  ];
}
```