Package apt: Information

- Made a bit nicer and more informative the new Debug::pkgMarkInstall messages,
  which were introduced in 0.5.15lorg2-alt81; renamed and mentioned there
  the new options: Debug::pkgMark-shallow, Debug::pkgMark-allcalls.
- Worked around the problem with printing long messages by making the buffer
  1200 bytes large. (ALT#44941)
- Avoid installing extra packages in some cases when this is really not needed
  to satisfy a request. (Correct cleanup in pkgProblemResolver::DoUpgrade.)

- tests:
  + Enhanced to be able to work with test packages containing some files.
    (This will be useful for testing the work with large RPM archives.)
  + Shortened the run time of apt-xxtra-heavy-load-checkinstall.
  + Shortened the run time of other *-checkinstall scripts
    (by shortening the the test with zillion packages).

- Fixed bad behavior when satisfying "Conflicts" or "Obsoletes" dependencies
  (discovered in ALT#42415), namely:
  + "Obsoletes" causing the deletion of its "virtual" targets (i.e.,
    providing packages) whereas this behavior is expected for Conflicts only;
  + versioned "Conflicts" (or "Obsoletes") causing the deletion of packages
    with non-matching version.
- Enriched the output of Debug::pkgMarkInstall with the versions of
  the dependency targets and the targets being considered during the search.
- Complemented it with new options (Debug::pkgMarkAllCalls,
  Debug::pkgMarkShallow) -- to understand better why the result of
  resolving a broken dep is unexpected in some cases.