openAbsolute should strip out the volume name on Windows

compiler/resolver.go

@@ -108,6 +108,13 @@ func (resolver *Resolver) ResolveCwd (address entity.Address) (string, error) {
 // paths, which the FSPL compiler runs on. It converts an absolute path to a
 // slash path relative to "/" and opens the file.
 func openAbsolute (filesystem fs.FS, path string) (fs.File, error) {
-	path = strings.TrimPrefix(filepath.ToSlash(path), "/")
+	rootName := "/"
+	// FIXME: this does not distinguish between volumes on windows. this is
+	// a limitation of os.DirFS, we might want our own DirFS implementation
+	// that has volumes as subdirs of /.
+	if volumeName := filepath.VolumeName(path); volumeName != "" {
+		rootName = volumeName
+	}
+	path = strings.TrimPrefix(filepath.ToSlash(path), rootName)
 	return filesystem.Open(path)
 }