POT Scan: Update for sequentialcreditnotes, eventcart, flexmailer, etc

Over the past half-year, there have been a few new folders within the civicrm-core file-hierarchy, e.g.

setup
ext/sequentialcreditnotes
ext/flexmailer
ext/eventcart
ext/ewaysingle
ext/financialacls
ext/greenwich
ext/afform
ext/search

The POT scanner should check these folders. Eventually, we're going to notice missing strings. There are mitigating factors - e.g. the corpus of strings isn't huge, and a lot of the strings existed before (either in different core folders or in a standalone extension), so there may be a bit of lag-time between the file-reorg and the appearance of symptoms.

There are currently two distinct localization flows -- iirc:

• The core flow - which splits strings into multiple POT files (contribute.pot, event.pot, etc), sends those to transifex, gets the PO's back out, and recombines into one MO (civicrm.mo). To capture the updated folders in this flow, we'd need to update create-pot-files.sh.
• The ext flow - which scans multiple repos, creating one POT per repo, sends those to transifex, gets the PO's back out, and publishes multiple MOs. To capture the updated folders in this flow, we'd need to update civiextensions-update-transifex.php and/or create-pot-files-extensions.sh and/or the scheduled-job.