Handle module abbreviations better

[safesparrow]

Currently when we see a module, we decide that the given file depends on all the previous files and all the files above it depend on it:

fsharp/tests/ParallelTypeCheckingTests/Code/DependencyResolution.fs

Lines 161 to 163 in ae0e40b

	// Assume that a file with module abbreviations can depend on anything
	match node.Data.Abbreviations |> Array.isEmpty |> not with
	| true -> nodes |> Array.map (fun n -> n.File)

This is a safety we need without fully understanding the behaviour and implementing it.

This ticket is to avoid this defensive mechanism.

Also: module abbreviations in signature files are currently not supported, we should fix it:

fsharp/tests/ParallelTypeCheckingTests/Code/ASTVisit.fs

Line 1147 in ae0e40b

failwith "Module abbreviations are not currently supported"

[safesparrow]

This is being handled in #24