Don't include leading underscores in implicit namespaces (#955)

Closes #934
See sass/sass#2800

Author: nex3

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 1.26.0-test.1
 
+* `@use` rules whose URLs' basenames begin with `_` now correctly exclude that
+  `_` from the rules' namespaces.
+
 * Don't throw errors if the exact same member is loaded or forwarded from
   multiple modules at the same time.
 

lib/src/parse/stylesheet.dart

@@ -1382,7 +1382,8 @@ relase. For details, see http://bit.ly/moz-document.
 
     var basename = url.pathSegments.isEmpty ? "" : url.pathSegments.last;
     var dot = basename.indexOf(".");
-    var namespace = basename.substring(0, dot == -1 ? basename.length : dot);
+    var namespace = basename.substring(
+        basename.startsWith("_") ? 1 : 0, dot == -1 ? basename.length : dot);
     try {
       return Parser.parseIdentifier(namespace, logger: logger);
     } on SassFormatException {