Core: Fix static dirs targeting same destination

[kroeder]

Issue: #16732

What I did

The previous implementation caused a race-condition if two assets target the same destination

  staticDirs: [
    '../src/assets',
    // reproduction of https://github.com/storybookjs/storybook/issues/16732
    { from: '../src/assets/a', to: 'assets' },
    { from: '../src/assets/b', to: 'assets/b' },
  ],

With this change, one staticDir after another gets copied without conflicting in a race condition

How to test

• Checkout this branch
• Revert my "fix" (there is a separate commit for the reproduction)
• Go to "examples/angular-cli" and run yarn build-storybook

The error in the mentioned issue should appear (mkdir file already exists)

[kroeder]

@ndelangen eslint is very unhappy the way I solved this

Though. I don't see the point here. The function above uses

await Promise.all( staticDirs.map( async (dir) => ... ));

which creates a map of promises and awaits them (technically also a loop with awaits in it?)
I even tried to use Promise.all but the error was not solved (which I also did not understand 🤷‍♂️ )

I targeted "main" because of the restructuring in "next". I was not able to build the angular-cli example