Feature Request: Get the modules defined in a given file

[axelson]

In the ElixirLS debugger it would be very helpful to get the list of modules defined in a file. This would allow the user to only interpret some of the modules in their project instead of all modules of the system. Defining by filename would be a more natural interface than a list of module names.

Related ElixirLS issue: https://github.com/elixir-lsp/elixir-ls/issues/615

Although after thinking about it in #615 I opted to go with a regex based on the module name I still think that this could be a useful feature for ElixirSense to have.