This implements the override pattern for builds done with buildEnv, so
that we can, for example, write

php.override { fpmSupport = false; }

and get a PHP package with the default extensions enabled, but PHP
compiled without fpm support.

Make all arguments to a PHP build overridable; i.e, both configuration
flags, such as valgrindSupport, and packages, such as valgrind:

php.override { valgrindSupport = false; valgrind = valgrind-light; }

This applies to packages built by generic and buildEnv/withExtensions;
i.e, it works with both phpXX and phpXXBase packages.

The following changes were also made to facilitate this:

 - generic and generic' are merged into one function

 - generic now takes all required arguments for a complete build and
   is meant to be called by callPackage

 - The main function called from all-packages.nix no longer takes all
   required arguments for a complete build - all arguments passed to it
   are however forwarded to the individual builds, thus default
   arguments can still be overridden from all-packages.nix

Make buildEnv take earlier overridden values into account by
forwarding all arguments (a merge of generic's arguments, all previous
arguments and the current arguments) to the next invocation of
buildEnv.

Rework withExtensions / buildEnv to handle currently enabled
extensions better and make them compatible with override. They now
accept a function with the named arguments enabled and all, where
enabled is a list of currently enabled extensions and all is the set
of all extensions. This gives us several nice properties:

 - You always get the right version of the list of currently enabled
   extensions

 - Invocations chain

 - It works well with overridden PHP packages - you always get the
   correct versions of extensions

As a contrived example of what's possible, you can add ImageMagick,
then override the version and disable fpm, then disable cgi, and
lastly remove the zip extension like this:

{ pkgs ? (import <nixpkgs>) {} }:
with pkgs;

let
  phpWithImagick = php74.withExtensions ({ all, enabled }: enabled ++ [ all.imagick ]);

  phpWithImagickWithoutFpm743 = phpWithImagick.override {
    version = "7.4.3";
    sha256 = "wVF7pJV4+y3MZMc6Ptx21PxQfEp6xjmYFYTMfTtMbRQ=";
    fpmSupport = false;
  };

  phpWithImagickWithoutFpmZip743 = phpWithImagickWithoutFpm743.withExtensions (
    { enabled, all }:
      lib.filter (e: e != all.zip) enabled);

  phpWithImagickWithoutFpmZipCgi743 = phpWithImagickWithoutFpmZip743.override {
    cgiSupport = false;
  };
in
  phpWithImagickWithoutFpmZipCgi743

Since all options controlled by the config.php parameters can now be
overridden directly, there's no reason to keep them around.

Some extensions depend on other extensions. Previously, these had to
be added manually to the list of included extensions, or we got a
cryptic error message pointing to strings-with-deps.nix, which wasn't
very helpful. This makes sure all required extensions are included in
the set from which textClosureList chooses its snippets.

This is useful if you need to access the dev output of the unwrapped
derivation.

Since the introduction of php.unwrapped there's no real need for the
phpXXbase attributes, so let's remove them to lessen potential
confusion and clutter. Also update the docs to make it clear how to
get hold of an unwrapped PHP if needed.

Instead of using two different php packages in php-packages.nix, one
wrapper and one unwrapped, simply use the wrapper and use its
"unwrapped" attribute when necessary. Also, get rid of the packages
and extensions attributes from the base package, since they're no
longer needed.

php.buildEnv: Make the exported php package overridable, improve handling of currently enabled extensions, etc