Fix support for BelongsToThrough relationships in Select/SelectFilter

[dchatry]

• Changes have been thoroughly tested to not break existing functionality.
  See examples below.
• New functionality has been documented or existing documentation has been updated to reflect changes.
  No need for docs changes, works as documented.
• Visual changes are explained in the PR description using a screenshot/recording of before and after.
  No visual changes.

This PR fixes support for staudenmeir/belongs-to-through relationships which used to be (at least partially) supported in V2.

Changes in packages/forms/src/Components/Select.php handle the proper key retrieval for this type of relationship for select population and searching.

A distinct() was added to the relationship query in packages/tables/src/Filters/SelectFilter.php to de-duplicate values used to build indicator labels.

Before:

After:

[danharrin]

Thanks!