Warn when an import shadows a locally defined function

[fcasal]

We would flag the following foo import

# library.cairo

func foo() -> ():
end

and

# main.cairo
from library import foo as bar

func bar() -> ():
end

func main() -> ():
    bar()
end

This should also work for unaliased import.

We should also check the compiler behavior so we could provide further details when this is found.

[coolhill]

The compiler seem to cover this case, both for aliased/unaliased imports.

$ cairo-compile main.cairo
main.cairo:3:6: Redefinition of '__main__.bar'.
func bar() -> ():
     ^*^