Turns out eyebrowse and neotree conflict a little when switching back
and forth between workspaces that are showing and not showing
neotree. So back to escreen it is for now.
- Move most logic from init.el into core/siren-init.el.
- Move siren-modules.el and siren-theme.el into core/ directory.
- Create core/siren-vendor.el to deal with setting up the vendor load
paths.
- Let core/siren-modules.el and core/siren-theme.el deal with setting
up their own relevant load paths.
