Add support for loading custom formatter from package

.gitignore

@@ -4,3 +4,4 @@ node_modules
 .eslintcache
 yarn.lock
 .vscode/settings.json
+.idea

lib/__tests__/resolveCustomFormatter.test.js

@@ -0,0 +1,35 @@
+const fs = require('fs');
+const path = require('path');
+const { resolveCustomFormatter } = require('../resolveCustomFormatter');
+
+jest.mock('fs');
+jest.mock('path');
+
+describe('resolveCustomFormatter', () => {
+	it('should return absolute path when provided path is a file path', () => {
+		const aRelativePath = 'a/relative/path';
+		const expected = `/cwd/${aRelativePath}`;
+
+		path.resolve.mockReturnValue(expected);
+		path.isAbsolute.mockReturnValue(false);
+		fs.existsSync.mockReturnValue(true);
+
+		const result = resolveCustomFormatter(aRelativePath);
+
+		expect(result).toEqual(expected);
+	});
+
+	it('should return provided path when path is neither absolute nor relative', () => {
+		const aModulePath = '@stylelint/prettier-config/index.js';
+
+		path.isAbsolute.mockReturnValue(false);
+		fs.existsSync.mockReturnValue(false);
+
+		const result = resolveCustomFormatter(aModulePath);
+
+		const realPathModule = jest.requireActual('path');
+		const expectedPath = realPathModule.join('node_modules', aModulePath);
+
+		expect(result).toEqual(expect.stringMatching(RegExp(`${expectedPath}$`)));
+	});
+});

lib/cli.js

@@ -13,6 +13,7 @@ const printConfig = require('./printConfig');
 const resolveFrom = require('resolve-from');
 const standalone = require('./standalone');
 const writeOutputFile = require('./writeOutputFile');
+const { resolveCustomFormatter } = require('./resolveCustomFormatter');
 
 const EXIT_CODE_ERROR = 2;
 
@@ -338,9 +339,7 @@ module.exports = async (argv) => {
 	let formatter = cli.flags.formatter;
 
 	if (cli.flags.customFormatter) {
-		const customFormatter = path.isAbsolute(cli.flags.customFormatter)
-			? cli.flags.customFormatter
-			: path.join(process.cwd(), cli.flags.customFormatter);
+		const customFormatter = resolveCustomFormatter(cli.flags.customFormatter);
 
 		formatter = require(customFormatter);
 	}

lib/resolveCustomFormatter.js

@@ -0,0 +1,20 @@
+const fs = require('fs');
+const path = require('path');
+
+/**
+ * @param {string} formatterPath
+ * @returns {string}
+ */
+function resolveCustomFormatter(formatterPath) {
+	const resolvedPath = path.resolve(formatterPath);
+
+	if (fs.existsSync(resolvedPath)) {
+		return resolvedPath;
+	}
+
+	return require.resolve(formatterPath);
+}
+
+module.exports = {
+	resolveCustomFormatter,
+};