Don't clone dkan for moduledev init if already there

[dafeder]

https://github.com/GetDKAN/dkan-ddev-addon/blob/e092657465b1752103d66bc91dbb49590718e58a/commands/host/dkan-init#L47

Check if /dkan already exists. If it does, don't run git clone.

Also, we should detect the version constraint from the git repo. See

https://github.com/GetDKAN/dkan-tools/blob/master/src/Command/InitCommands.php#L243

for how this was done in dkan-tools

[paul-m]

Because of the way ddev delights in demolishing your project directory if you say ddev composer create (which we do), I can either figure out how and what to backup before we do this, or we can just warn the user and abort if there's anything in the directory that might get destroyed.

Let's postpone here and work on #9 instead, which addresses the issue of not demolishing anything during dkan-init.

After that we can swing back through this issue and ensure that we can specify a branch for moduledev.

[paul-m]

Un-postponing in support of WCMS-11097