The new layer file `layers.el` is used to declared additional layers.
It is like the sibling of packages.el except that for now it does not
take a list (can do this in a futur commit).
The new order for file loading is the following:
layers.el > packages.el > funcs.el > config.el > keybindings.el
Since packages.el relies on some undefined stuff encapsulated
in init functions, it is not meant to be byte compiled. OTOH funcs.el
(where lies most of the computation added by a package config) should
be compilable.
Since we load packages.el very early it is not possible to use
`configuration-layer/package-usedp` in funcs.el.
This commit also fixes the tests.
Change the semantic of the files for a better control over the
configuration (like being able to use package-usedp function to
guard package configuration and function definition).
This is a big change!
The list of packages of the layer must now be placed in config.el
packages.el only contains pre-init, init and post-init functions
Now both packages.el and funcs.el can use
configuration-layer/package-usedp which is much more powerful than
just configuration-layer/layer-usedp.
Also merging into one file the layer variable and the list of packages
make sense (in this case config.el).
It also means that we can quickly load all the packages declared in
all layer for helm and ivy sources by just loading the config.el file.
Expect some regressions introduced by this change, like the helm
and ivy sources, they will be fixed in other commits.
