allow to skip duplicated files

[Dartui]

Problem

if you want to use two packages with same namespace (e.g. illuminate/support and illuminate/collections) which shares similar files, Mozart will throw an "File already exists" error.

Solution

While moving file check if it already exists. In case mentioned above it is only LICENSE.md file, so it doesn't matter from which package it will be. Also added flag to config to allow enabling this behavior (disabled by default).