This replaces the older pattern
:toggle (configuration-layer/package-usedp ..)
This implementation ensures that :disabled-for honors dependent packages, i.e.
if package a depends on package b, which is owned by layer c, and layer c is
disabled for layer d, then neither package a nor b will be configured for layer
d. Previously, this was only true for package a, but not b.
This commit also fixes:
- configuration-layer/describe-package now shows which post-init and pre-init
functions are disabled, if any
- Does not recreate all layer objects unconditionally when calling
configuration-layer/discover-layers. Previously, this led to all layers being
recreated after e.g. `SPC h SPC`, without any of the dotfile information.
Since this information is now necessary for
configuration-layer/describe-package, it’s important that we don’t clear the
indexed layers when invoking this function.
