Only suggest filters available in SourceIndex

[Poitrin]

WHY are these changes introduced?

Relates to #656 and #674, rolls back changes to filters suggestion.

WHAT is this pull request doing?

• Filters that were previously in data/shopify_liquid/filters.yml will not be suggested.
• Theme Check will accept these filters though.

How to test your changes?

• Verify that code completion for tags is still working as before,
  • and that the tags mentioned in LABELS_NOT_IN_SOURCE_INDEX keep being suggested.
• Verify that code completion for objects is still working as before,
  • and that customer_address and product_variant keep being suggested, but offer no further suggestions like attribute suggestions.
• ⚠️ Verify that the filters mentioned in LABELS_NOT_IN_SOURCE_INDEX are not suggested anymore.

[karreiro]

Thank you, @Poitrin!