proc_macro_plugin: Wrap nonexistent filename in <>

proc_macro_plugin: Wrap nonexistent filename in <>

I'm not sure how big of an issue this can become in practice, but `FileMap`s made from something that's not a file are supposed to wrap the file name in `<>`.

For an example fix, see kevinmehall/rust-peg@332fd4dbae19b64fb2af4b9d565e37a3703e7576. There, it caused cargo to always recompile a crate using rust-peg, even when nothing was changed, because cargo sees that the dummy file doesn't exist.