Extensions: Improve logic to lookup script extensions

- Use identical logic for looking up <dependency>s and <command>s.
- Remove duplicate (but inconsistent and incomplete) logic from
  script.cpp that was used to search for the <command> again
- Remove <check> element from .inx format
  It seems unused (at the very least by core extensions) and
  redundant to <dependency> checking
- Deprecate the <command>-specific "reldir" attribute.
  Consistently use the functionally identical "location" attribute
  that was only used for <dependency>s before
- Introduce the new relative location value location="inx",
  which looks up <dependencies> and <command>s relative to the
  .inx file's location.

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