css: resolve bare module specifiers in @import to node_modules (#485)

Enable node_modules fallback resolution for CSS @import, matching
the existing behavior in SCSS and LESS. Also fix getModuleNameFromPath
to return the full path for bare specifiers without a slash (e.g.
@import "bootstrap") instead of returning an empty string.

Fixes microsoft/vscode#295074

Signed-off-by: Vijay Govindarajan <vijay.govindarajan91@gmail.com>

Author: vijaygovindaraja

src/cssLanguageService.ts

@@ -116,7 +116,7 @@ export function getCSSLanguageService(options: LanguageServiceOptions = defaultL
 		new Parser(),
 		new CSSCompletion(null, options, cssDataManager),
 		new CSSHover(options && options.clientCapabilities, cssDataManager),
-		new CSSNavigation(options && options.fileSystemProvider, false),
+		new CSSNavigation(options && options.fileSystemProvider, true),
 		new CSSCodeActions(cssDataManager),
 		new CSSValidation(cssDataManager),
 		cssDataManager

src/services/cssNavigation.ts

@@ -593,7 +593,7 @@ function toTwoDigitHex(n: number): string {
 export function getModuleNameFromPath(path: string) {
 	const firstSlash = path.indexOf('/');
 	if (firstSlash === -1) {
-		return '';
+		return path;
 	}
 
 	// If a scoped module (starts with @) then get up until second instance of '/', or to the end of the string for root-level imports.

src/test/css/navigation.test.ts

@@ -460,6 +460,16 @@ suite('CSS - Navigation', () => {
 				[{ range: newRange(8, 24), target: getTestResource('green/c.css') }], 'css', testUri, workspaceFolder
 			);
 		});
+
+		test('bare module specifier resolving', async function () {
+			const ls = getCSSLS();
+			const testUri = getTestResource('about.css');
+			const workspaceFolder = getTestResource('');
+
+			await assertLinks(ls, '@import "foo/bar.css"',
+				[{ range: newRange(8, 21), target: getTestResource('node_modules/foo/bar.css') }], 'css', testUri, workspaceFolder
+			);
+		});
 	});
 
 	suite('Color', () => {