This data structure was added in 32f8727,
but its functionality has since been provided by LoweredCodeUtils and is
no longer needed. This removes all code related to `moduledeps` and clean up.

Separate functions based on what they return.

The interface added in #680 for supporting non-.jl
files assumes usages of custom struct, which requires overloading
functions like `abspath` and is therefore quite cumbersome.
Worse still, that makes it harder to refactor Revise code base, since
now we need to think about the possibility of arbitrary types in places
that originally needed to handle `AbstractString`.

This commit provides a simpler interface: support custom files by
registering parsers per file extension in
`Revise.parsers::Dict{String,Any}`.
The greatly simplified `non_jl_test.jl` demonstrates how much more
straightforward this approach is.
Of course, a potential downside is that if multiple packages register
different parsers for the same extension, conflicts could arise, but
since this feature is experimental, I believe we don’t need to account
for that scenario yet.

Remove interface for supporting non-.jl files introduced in #680.
This interface forces functions that used to handle only `AbstractString`
to deal with arbitrary types, making refactoring across the Revise.jl
codebase difficult.
The simplified interface proposed in the previous commit is also
fragile since it relies on initialization and runtime-mutable data
structures, and so it is therefore not a complete solution.
After discussion, and since this experimental interface has effectively
no users, we remove it for now to enable codebase refactoring until
it becomes necessary again.

remove the interface to support non .jl files

Separated from the refactoring I'm working on locally.