Add 2-arg include mapexpr to cache header?

[timholy]

By my reading, a package file loaded with include(mapexpr, filename) does not preserve mapexpr in the cache header. This makes it hard for Revise to handle 2-arg include without defensively parsing the whole package, checking to see if there are any 2-arg includes.

For Revise to handle 2-arg include, at a minimum we probably need to annotate the header to say "this file was loaded with a 2-arg include," even if we don't store any information about mapexpr itself. This will allow Revise to skip defensive package parsing for the large majority of packages.

xref timholy/Revise.jl#634