fix(modules): use prettier-eslint versions of modules if needed

This way you can install prettier-eslint and not have
prettier or eslint installed yourself and it should
still work.

package.json

@@ -52,7 +52,7 @@
     }
   },
   "jest": {
-    "testEnvironment": "node",
+    "testEnvironment": "jest-environment-node",
     "coverageThreshold": {
       "global": {
         "branches": 100,

src/__mocks__/eslint.js

@@ -1,5 +1,5 @@
 // this mock file is so eslint doesn't attempt to actually
-// search around the filesystem for stuff
+// search around the file system for stuff
 
 const eslint = require.requireActual('eslint')
 const {CLIEngine} = eslint
@@ -17,9 +17,14 @@ module.exports = Object.assign(eslint, {
 })
 
 function MockCLIEngine(...args) {
+  global.__PRETTIER_ESLINT_TEST_STATE__.eslintPath = __filename
   CLIEngine.apply(this, args)
+  // not sure why, but in some cases, this.executeOnText is undefined...
+  // so we create a fakeCLIEngine to get a copy of that function
+  // and call it with apply :)
+  const fakeCLIEngine = new CLIEngine(...args)
   this.getConfigForFile = mockGetConfigForFileSpy
-  this._originalExecuteOnText = this.executeOnText
+  this._originalExecuteOnText = fakeCLIEngine.executeOnText
   this.executeOnText = mockExecuteOnTextSpy
 }
 

src/__mocks__/prettier.js

@@ -10,6 +10,7 @@ Object.assign(prettier, {
 })
 
 function mockFormat(...args) {
+  global.__PRETTIER_ESLINT_TEST_STATE__.prettierPath = __filename
   if (mockFormatSpy.throwError) {
     throw mockFormatSpy.throwError
   }

src/index.js

@@ -120,7 +120,12 @@ function getConfig(filePath, eslintPath) {
 }
 
 function getModulePath(filePath = __filename, moduleName) {
-  return requireRelative.resolve(moduleName, filePath)
+  try {
+    return requireRelative.resolve(moduleName, filePath)
+  } catch (error) {
+    logError(`There was a problem finding the ${moduleName} module. Using prettier-eslint's version`, error.stack)
+    return require.resolve(moduleName)
+  }
 }
 
 function getESLintCLIEngine(eslintPath, eslintOptions) {

src/index.test.js

@@ -203,6 +203,18 @@ test('resolves to the eslint module relative to the given filePath', () => {
   })
 })
 
+test('resolves to the local eslint module if none is found via the filePath', () => {
+  const filePath = '/blah-blah/default-config'
+  format({text: '', filePath})
+  expect(global.__PRETTIER_ESLINT_TEST_STATE__).toMatchObject({
+    // without Jest's mocking, these would actually resolve to the
+    // project modules :) The fact that jest's mocking is being
+    // applied is good enough for this test.
+    eslintPath: require.resolve('./__mocks__/eslint'),
+    prettierPath: require.resolve('./__mocks__/prettier'),
+  })
+})
+
 function getESLintConfigWithDefaultRules(overrides) {
   return {
     parserOptions: {