* Ignore top-level module config; add auto-update
In Spack 0.17 we got module sets (modules:[name]:[prop]), and for
backwards compat modules:[prop] was short for modules:default:[prop].
But this makes it awkward to define default config for the "default"
module set.
Since 0.17 is branched off, we can now deprecate top-level module config
(that is, just ignore it with a warning).
This PR does that, and it implements `spack config update modules` to
make upgrading easy (we should have added that to 0.17 already...)
It also removes references to `dotkit` stuff which was already
deprecated in 0.13 and could have been removed in 0.14.
Prefix inspections are the only exception, since the top-level prefix inspections
used for `spack load` and `spack env activate`.
99083f1706