Update tutorial on advanced packaging (#7144)

This reorganizes most sections and rewords a significant portion of
the content (including all introductions) but keeps all the examples.

* Remove section 'What happens at subscript time' from tutorial:
  it is too detailed for a tutorial
* Move the 'Extra query parameters' and 'Attach attributes to other
  packages' sections into a separate grouping 'Other packaging topics'
* move the 'Set variables at build time yourself' section after
  'Set environment variables in dependents' section since the latter
  is more motivating
* start the 'set environment variables at build-time for yourself'
  section with qt as an example
* renamed section 'specs build interface' to 'retrieving library
  information' and updated section introduction
* renamed section 'a motivating example' to 'accessing library
  dependencies'; split out the material which deals with implementing
  .libs for netlib-lapack into a separate section called 'providing
  libraries to dependents'. consolidated in material from the section
  'single package providing multiple virtual specs' since
  netlib-lapack is an example of this (this removes the material
  about intel-parallel studio)

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