Suggestion: Use Different ELPA Directories for Different Emacs Versions

[bmag]

Using the same .emacs.d/elpa/ directory for different versions of Emacs causes errors, since the .elc files of one version can be incompatible with other other versions. I propose to set the package-user-dir in core-configuration-layer.el according to the variable emacs-version. Instead of a single elpa directory, we will have elpa/24.3, elpa/24.4, etc. The end result is to be able to use the same .emacs.d directory for different Emacs versions.
This probably requires some other changes to the core and to the tests as well.

[StreakyCobra]

Pending PRs: #3384 #4312

[d12frosted]

Fixed with released of Spacemacs v0.200. Let us know if you still have any problems with this issue.