How to add dependency to a custom module with drupal-composer?
Ok, seems to me that I almost got it. 

This was due to my repository list in the Drupal root composer.json. I had this kind of thing there:

{
  "type": "package",
  "package" : {
    "name": "blaah",
    "version" : "xxx",
    "type":"drupal-module",
    "source": {
      "url": "gitblaag",
      "type": "git"
    }
}


That some how seems to override what ever is in the modules composer.json. I do not really know why but something along the lines that the entry defines the whole package and no more info will be read from the composer.json in the module. 

So I adding "require" there does the magic. Also probably removing some of those lines and making it just a repository, not a package, would result into composer reading the individual modules composer.json.

But now everything seems to be right and module is installed and it will install my other files under vendor but PHP still goes to "class not found" in my service when I try to use my class from the other library.