extensions: fix extensions handling

[drupol]

Listing available extensions for a PHP derivations is currently broken.

Reasons are:

1. Syntax errors
2. Wrong way of handling extensions
3. Missing upstream extension, not defined in here

Therefore, the idea behind this PR is to be able to list all the available extensions for any php derivations, without errors.

Doing (replace XX with the PHP version):

NIXPKGS_ALLOW_BROKEN=1 \
NIXPKGS_ALLOW_INSECURE=1 \
NIXPKGS_ALLOW_UNFREE=1 \
nix eval --json --impure --expr '(import ./.).packages.x86_64-linux.phpXX.extensions'

should not throw any error, just list the available extensions.

This PR:

• Fix Trying to list all extensions of a php derivation #245