Normalize fable-library argument

[nojaf]

Fixes #3653

The problematic code is, I believe, that

Fable/src/Fable.Transforms/Global/Prelude.fs

Lines 394 to 427 in 38c8e10

	let getRelativeFileOrDirPath
	fromIsDir
	(fromFullPath: string)
	toIsDir
	(toFullPath: string)
	=
	// Algorithm adapted from http://stackoverflow.com/a/6244188
	let pathDifference (path1: string) (path2: string) =
	let mutable c = 0 //index up to which the paths are the same
	let mutable d = -1 //index of trailing slash for the portion where the paths are the s
	while c < path1.Length && c < path2.Length && path1.[c] = path2.[c] do
	if path1.[c] = '/' then
	d <- c
	c <- c + 1
	if c = 0 then
	path2
	elif c = path1.Length && c = path2.Length then
	String.Empty
	else
	let mutable builder = ""
	while c < path1.Length do
	if path1.[c] = '/' then
	builder <- builder + "../"
	c <- c + 1
	if builder.Length = 0 && path2.Length - 1 = d then
	"./"
	else
	builder + path2.Substring(d + 1)

expects the paths already to be in / notation.
Converting this earlier on will ensure the relative paths in the import statements are fixed.

Also, trying out Windows CI again for JS. It worked on my machine.

[MangelMaxime]

Thank you