Use module name as the input file name if the latter isn't provided (i.e., code via stdin)

[sile]

Before

$ echo "-module(foo). \nfoo -> ok." | efmt -
[2022-08-18T11:51:00Z ERROR efmt] Failed to format "-"
    Parse failed:
    --> <unknown>:2:5
    2 | foo -> ok.
      |     ^ unexpected token

After

$ echo "-module(foo). \nfoo -> ok." | efmt -
[2022-08-18T11:50:23Z ERROR efmt] Failed to format "-"
    Parse failed:
    --> foo.erl:2:5
    2 | foo -> ok.
      |     ^ unexpected token