There was a edge case with the declaration of the `lsp` layer in `layers.el`
files.
The `hy` layer depends on the `python` layer which in turn depends on the `lsp`
layer if and only if the `python-backend` layer variable is set to `lsp`.
When the `hy` layer was declared first then it declares the `python` layer
without its layer variables, thus the `lsp` layer was not declared because the
`python-backend` variable was not set.
The fix is to gather all the layer dependencies and resolve them only after all
the used layers have been declared.
* new function `configuration-layer/declare-layer-dependencies`
* replace all calls to `configuration-layer/declare-layer` by the new function
except for distribution layers (we declare layer dependencies right away in
distribution layers)
- Move rtags key bindings under `SPC m g` prefix
- Document rtags support and key bindings in README.org
- attempt to implement gtags fallback using
https://github.com/Andersbakken/rtags#fall-back-to-other-taggers
- make c-c++ layer dependend on gtags layer (gtags layer is automatically
installed when the c-c++ layer is used)
- sort packages config in packages.el
- use a dolist instead of a function to define rtags key bindings
