Add warnings for unused imported symbols

[yorickpeterse]

Description

When defining unused local variables, the compiler emits a warning. We should do the same for any unused symbols imported using the import statement.

I'm not entirely sure yet how to best implement this, but I think it should be something along the lines of this:

When lowering to MIR, we keep track of used symbols using a HashMap<types::Symbol, (LocationId, bool)> (one such map per module, not per method).

The LocationId is the MIR location ID at which the symbol import is defined (= the location at which the imported symbol is listed, i.e. the foo in import std.whatever (foo)).

The bool defaults to false and is set to true when referred to in a reachable block.

At the end of a module we iterate over this map and add warnings for every Symbol where the bool is false, using the LocationId to retrieve the line/column position, and using the current module file as the source file.

Related work

No response

[yorickpeterse]

Note that when a symbol is aliased as part of an import, the LocationId should point to the location of the alias, instead of the location of the original symbol name.