ESLint: Use custom module resolver for ESLint

This custom resolver inspects the file's path to decide which off-the-shelf resolver would be appropriate. For files from the client folder, this resolver uses Webpack's resolver. For other files, this resolver uses Node's resolver.

.eslint-resolver.js

@@ -0,0 +1,55 @@
+/**
+ * External dependencies
+ */
+const assign = require ( 'lodash/assign' );
+const debugFactory = require( 'debug' );
+const nodeResolver = require( 'eslint-import-resolver-node' );
+const path = require( 'path' );
+const webpackResolver = require( 'eslint-import-resolver-webpack' );
+
+/**
+ * Internal dependencies
+ */
+const webpackResolveConfig = require( './webpack.config.resolve' );
+
+/**
+ * Module variables
+ */
+const log = debugFactory('eslint-plugin-import:resolver:wp-calypso-resolver');
+
+const nodeResolverConfig = {
+	extensions: [
+		'.js',
+		'.jsx',
+	],
+	moduleDirectory: [
+		'node_modules',
+		'server'
+	],
+	paths: [
+		path.join( __dirname, 'test' ),
+		path.join( __dirname, 'server' ),
+		path.join( __dirname, 'client' ),
+	]
+};
+
+const webpackResolverConfig = {
+	config: {
+		resolve: webpackResolveConfig,
+	},
+};
+
+exports.interfaceVersion = 2
+
+exports.resolve = function (source, file, config) {
+	log('Resolving', source, 'from:', file);
+	// If the file is from the client folder, use Webpack for resolution.
+	// Otherwise, use Node's resolver.
+	if (file.indexOf('/client/') > 0) {
+		log('Client file detected, resolving with Webpack');
+		return webpackResolver.resolve( source, file, webpackResolverConfig );
+	} else {
+		log('Resolving with Node resolver');
+		return nodeResolver.resolve( source, file, nodeResolverConfig );
+	}
+}

.eslintrc.js

@@ -1,5 +1,7 @@
 /* eslint-disable import/no-commonjs */
-const resolveConfig = require( './webpack.config.resolve.js' );
+
+const path = require( 'path' );
+const customResolverPath = path.join( __dirname, './.eslint-resolver.js' );
 
 module.exports = {
 	root: true,
@@ -17,21 +19,14 @@ module.exports = {
 		'import'
 	],
 	settings: {
-		'import/ignore': [
-			'server\/.*$',
-		],
 		'import/resolver': {
-			webpack: {
-				config: {
-					resolve: resolveConfig,
-				}
-			},
+			[ customResolverPath ]: {}
 		}
 	},
 	rules: {
 		camelcase: 0, // REST API objects include underscores
-		// NOTE: Some import rules are errors in client and warnings in server
-		//       due to how CommonJS and ES module notations inter-op.
+		// NOTE: Some import rules are errors in client (Webpack's resolution) and
+		//       warnings in server (Node's module resolution).
 		'import/default': 2,
 		'import/export': 2,
 		'import/named': 2,