fix and simplify Dpkg::Vendor::PureOS as minimal subclass

Dpkg::Vendor::PureOS was created as a copy of Dpkg::Vendor::Debian,
borrowing parts of Dpkg::Vendor::Ubuntu, and adding some unique bits.

  * borrowed code contains bogus/needless references to Ubuntu
  * borrowed code contains no-op calls to SUPER
    (because it was borrowed from a subclass)
  * code is hard to maintain going forward

This patch refactors Dpkg::Vendor::PureOS as a subclass of
Dpkg::Vendor::Debian (similar to Dpkg::Vendor::Ubuntu), fixes a warming
to reference PureOS (not Ubuntu), drops unneeded Ubuntu-specific parts,
and only minimally overloads run_parts().
4 jobs for pureos/master in 0 seconds (queued for 70 minutes and 7 seconds)
latest
Status Job ID Name Coverage
  Test
failed #333790
dist-check

failed #333793
func-tests

failed #333791
unit-tests

failed #333792
vpath-tests

 
Name Stage Failure
failed
func-tests Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
vpath-tests Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
unit-tests Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
dist-check Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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