Use slash instead of backslash under Windows

[andreasabel]

I get not ok ... stdout verdicts under Windows when the same tests pass under Linux and macOS.
I'd guess these are caused by directory globbing resolved under Windows using backslashes instead of slashes, so printing the filepath to stdout gives different results under Linux and Windows. https://github.com/andreasabel/miniagda/blob/67f9ad914f8bbda153d679a517da98b01e482d4e/test/fail/adm/adm1.err#L1
Since Windows nowadays understands the slash as path separator, maybe the slash could be used instead of the backslash to make goldplate more portable?

[andreasabel]

It could be that the call to normalise here is to blame: https://github.com/fugue/goldplate/blob/5ca490ca7845417f7f67da5fb682001468e2862f/src/Main.hs#L222
According to the docs, it includes the following replacement:

/ -> pathSeparator

This means that under Windows, slashes become backslashes.