I found it fairly hard to trace through which source-code files mapped to
which *.pot files; there were several interacting rules/patterns in
create-pot-files.sh and in extractor.php which determined that policy
question.

This rewrite replaces *extractor.php with civistrings and moves all the
policy/pattern questions into create-pot-files.sh (with a big "case"
statement).  For example, the question of "What files go into event.pot?"
could be answered by this clause:

```
function build_raw_pot() {
  ...
  case "$name" in
    ...
    Event)
      _civistrings -o "$filepath" \
        CRM/Event \
        templates/CRM/Event \
        xml/templates/message_templates/event_* \
        xml/templates/message_templates/participant_*
      ;;
  esac
  ..
}
```

Any PHP/TPL/JS/HTML in those files/folders will be scanned for strings.