qmlls: rework completion codes for imports

Currently, the code for completions of import statements only suggests Modules that are loaded into the Dom. For example, you don't get any completions for
"import QtQuick.Controls.". If you add "import QtQuick.Controls.Basic" to the file, then suddenly you get "Basic" as a completion item when you try to autocomplete "import QtQuick.Controls." again.

Implement module name suggestions for all modules that qmlls can find.