fix: fix error occurred when cli copy files and add compatible logic

[GaoYYYang]

This PR contains a:

• bugfix

Motivation / Use-Case

I found some errors when i use webpack-defaults cli.
At first，when i run it in an empty directory(without existing package.json), it will fail。

/Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/dist/tasks/package.js:67
      defaults: existing.scripts.defaults || 'webpack-defaults'
                                 ^

TypeError: Cannot read property 'defaults' of undefined
    at module.exports (/Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/dist/tasks/package.js:67:34)
    at /Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/dist/index.js:18:5
    at Array.forEach (<anonymous>)
    at webpackDefaults (/Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/dist/index.js:15:9)
    at Object.<anonymous> (/Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/bin/index.js:3:19)
    at Module._compile (internal/modules/cjs/loader.js:1133:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1153:10)
    at Module.load (internal/modules/cjs/loader.js:977:32)
    at Function.Module._load (internal/modules/cjs/loader.js:877:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)

Second，after i overcome the first problem by creating a package.json file, error show up when copy files.

Create test/loader.test.js
(node:79169) UnhandledPromiseRejectionWarning: MrmError: copyFiles: source file not found: /Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/templates/test/options.test.js
    at /Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/node_modules/mrm-core/src/fs.js:32:10
    at Array.forEach (<anonymous>)
    at copyFiles (/Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/node_modules/mrm-core/src/fs.js:29:21)
    at /Users/bytedance/.nvm/versions/node/v12.16.3/lib/node_modules/@webpack-contrib/defaults/dist/tasks/templates.js:26:7
(node:79169) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:79169) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Then，when i excute npm test, prettier will exit status 1 like that :

> test@1.0.0 lint:prettier /Users/bytedance/Documents/work/code/awebpack/test
> prettier "{**/*,*}.js" --list-different

.eslintrc.js
.prettierrc.js
babel.config.js
commitlint.config.js
husky.config.js
lint-staged.config.js
src/cjs.js
src/index.js
test/helpers/normalizeErrors.js
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! test@1.0.0 lint:prettier: `prettier "{**/*,*}.js" --list-different`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the test@1.0.0 lint:prettier script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/bytedance/.npm/_logs/2020-06-29T13_39_42_161Z-debug.log

At last, i found it would't generator the script defaults in my local package.json

I have fixed all problems above.

Breaking Changes

Additional Info

[jsf-clabot]

All committers have signed the CLA.

[rgant]

Very Helpful, Thanks!