Need to ensure initial settings are applied before sourcing instant/*.vim

Plugin management code needs to do the following operations in order:
1. Check if plugin FOO is installed on runtimepath
2. If not: Fetch plugin, add to rtp
3. Source instant/flags.vim
4. Apply initial settings (e.g., `!instant[somefile]`)
5. Source other instant files

We currently have a problem that checking whether plugin FOO is installed on runtimepath (using `maktaba#plugin#IsRegistered` or similar) will automatically trigger step 5 before we've had a chance to get to step 4. This is bad because it makes it impossible to disable instant/ files before they're initially sourced.