Instead of manually just setting faces after loading a doom-themes
theme, let's use a custom override theme which we apply right after
applying any doom-themes theme.
The override theme uses various doom-color helpers, so the colors it
uses will be based on the most recently applied doom-themes theme.
I've been using doom-themes' doom-vibrant for long enough now that I'm
certain I won't be moving away from it anytime soon. So let's simplify
and strip away all other theme setup stuff.
The core setup files for Emacs Siren which lives in the core directory
followed a `siren-*.el` naming convention, which is the same as the
naming convention for modules.
This means that the `modules/core/siren-packages.el` module for adding
packages for Emacs package development, was not being loaded due to it's
name conflicting with `core/siren-packages.el` which sets up and
configures the packaging system.
So all files under the root `core` directory now follow a
`siren-core-*.el` naming scheme, meaning modules should no longer
conflict with core files.
