Fix unchanged revision hashes in Windows

Use `upath` package to make all file paths become in POSIX style.

lib/compile.js

@@ -1,4 +1,5 @@
 const path = require("path");
+const upath = require("upath");
 const url = require("url");
 
 const sass = require("sass");
@@ -9,6 +10,8 @@ const getLoadPathsFixedSassOptions = require("./get-load-paths-fixed-sass-option
 
 const debugDev = require("debug")("Dev:EleventySass:Compile");
 
+const workingDir = upath.normalize(process.cwd());
+
 const compile = async function(inputContent, inputPath, sassOptions, config, postcss) {
   let parsed = path.posix.parse(inputPath);
   if (parsed.name.startsWith("_")) {
@@ -55,7 +58,7 @@ const compile = async function(inputContent, inputPath, sassOptions, config, pos
   // become self-referential. This means that `dependantsOf()` for an updated file
   // returns not only files that depend on it but also the updated file itself.
   let dependant = path.posix.normalize(inputPath);
-  let loadedPaths = loadedUrls.map(loadedUrl => path.relative('.', url.fileURLToPath(loadedUrl)));
+  let loadedPaths = loadedUrls.map(loadedUrl => path.posix.relative(workingDir, url.fileURLToPath(loadedUrl, {windows: false })));
   depMap.update(dependant, loadedPaths);
 
   return css;