Relax the source requirement for source-definitions

[RFSH]

Currently while defining a "source" in the source-definitions annotation, you MUST have the source property. This means,

• You cannot use sourcekey to refer to other sources.
  • Work around: you have to copy/paste the same path multiple times.
  • Refer to this comment for more information.
• You cannot define virtual columns (columns without any source).
  • Work around: given that you've already defined the name and presentation value, you can get around this by defining a local column as the source. Although this will have some side effects that might not be desirable. For example,
    • The duplicate detection in visible columns will use the column name. so the column cannot already be part of your visible-columns
    • In compact contexts, the column will be sortable and it will sort based on the local column (not what’s displayed)
    • If you define this in a * context, the regular column will show up in entry contexts

So we should see how we might want to allow this. Usage of sourcekey in source-definitions came up before in #909.