`State_selection` has its signature embedded into signature `S`, shared
with submodule `Static`, marked as deprecated since 2012. If #1103 is
fixed, we can revert this commit.

With the current checks, indexing `M.s` where `s` is in fact
`include`d in `M`'s signature can't be done with
`\scodeidx{M}{s}`: you can only refer to `\codeidx{M}` and
`\scodeidx{I}{s}` where `I` is the module where `s` actually appears.

Changes seems to have taken place for 18.0-Argon according to the
comments on the relevant functions