Fix/facets tax query

[oscarssanchez]

Description of the Change

This PR fixes #2502 by checking if we have a pre existing tax query done by external code (in this case, the Woocommerce brands plugin). If we do, there's no need to add another tax query with EP if using facets.

Alternate Designs

Possible Drawbacks

Verification Process

1.- Have some brands and products associated with them
2.- Have a layared brand nav widget set
3.- Select one of the brands in the widget in the frontend
4.- Associated products should appear

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Credits

Props @

[felipeelia]

@oscarssanchez talked to @brandwaffle about this one and it would probably be better to change our "filter_" to "ep_filter_", so we avoid any future conflict that may arise. Obviously, that would be a filterable string, so people could go back to "filter_" if they want. What do you think?

[oscarssanchez]

Assigning for review @felipeelia