An empty entry in EMACSLOADPATH gets filled with the default value.
This is presumably why the wrapper inserted a colon after the entry it
added for the dependencies.  But this naive approach wasn't always
correct.

For example, if the user ran emacs with EMACSLOADPATH=foo, the wrapper
would insert the default value (by adding the trailing `:') even
though the user was trying to expressly opt out of it.

To do this correctly, here I've replaced makeWrapper with a bespoke
script that will actually parse the EMACSLOADPATH provided in the
environment (if given), and insert the wrapper's load path just before
the default value.  If EMACSLOADPATH is given but contains no default
value, we respect that and don't add the wrapped dependencies at all.
If no EMACSLOADPATH is given, we insert the wrapped dependencies
before the default value, just like before.  In this way, the wrapped
Emacs should now behave as if the wrapped dependencies were part of
Emacs's default load-path value.

If I'm running an Emacs executable from emacsWithPackages as my main
programming environment, and I'm hacking on Emacs, or the Emacs
packaging in Nixpkgs, or whatever, I don't want the Emacs packages
from the wrapper to show up in the load path of that child Emacs.  It
results in differing behaviour depending on whether the child Emacs is
run from Emacs or from, for example, an external terminal emulator,
which is very surprising.

To avoid this, pass another environment variable containing the
wrapper site-lisp path, and use that value to remove the corresponding
entry in EMACSLOADPATH, so it won't be propagated to child Emacsen.