See end of this message for important breaking changes.
Previous behavior was to configure any installed package which caused
a lot of bad side effects and could make spacemacs unusable. This
behavior made little sense and does not fit with spacemacs.
This commit fixes this behavior by separating installed packages from
configured packages. In short dostspacemacs-download-packages variable
now only affect package installation. Packages are now configured if and
only if they are effectively *used* (i.e. listed in variable
dotspacemacs-configuration-layers or dotspacemacs-additional-packages).
IMPORTANT CHANGE: functions `configuration-layer/declare-used-layer` and
`configuration-layer/declare-used-layers` have been removed. These
functions have been introduced in develop branch only so the impact
should be minimal.
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