Package apt: Information

- Invalidate the in-memory cache of repositories when doing "update" or
  ListUpdate() to be able to detect updates without exiting the process,
  e.g., PackageKit or apt-shell (ALT#41816).
- Added apt-under-pkdirect-checkinstall subpackage
  and added a test for FileList() API via packagekit.

- Implemented generic callback system for packagekit, allowing to show progress
  during offline-update to user. (Thx Oleg Solovyov mcpain@)

- (tests) Report if a test (marked XFAIL) uneXpectedly passes (XPASS).
- (tests) Run them not in %check, but in *-checkinstall subpkgs. (To break
  build-dep cycle with apt-repo-tools, whose features are required by the tests,
  but which needs to be recompiled to be linked with libapt.)
- (tests) Done more extensive testing of how apt works with "rpm" repos
  via any of the file, http(s), cdrom methods; including:
  + re-fetching if the saved complete or partial pkglist indices are corrupt
    (see https://bugzilla.altlinux.org/show_bug.cgi?id=40746#c9 );
  + the verification of the checksums of pkglist indices. (The verification
    is tested in two ways:
    * The verification of the checksum of a specific type is tested by faking
      it in the meta-data: for MD5, SHA1, SHA256, BLAKE2b and just the size.
    * Simply testing that a faked pkglist file of the same size is rejected--no
      matter which hashing algorithm is used.)
  + the verification of the checksums of rpm archives. (The verification
    is tested in two ways:
    * The verification of the checksum of a specific type is tested by faking
      it in the meta-data: for MD5, SHA1, SHA256, BLAKE2b.
    * Simply testing that a faked rpm file of the same size is rejected--no
      matter which hashing algorithm is used.)
- (source code; ABI) Reverted a lot of inessential optimizations
  from 0.5.15lorg2-alt72.
- (source code; ABI) Got rid of virtual methods with default parameters
  (because they are confusing for the programmer).
- (source code; ABI) Backported some pieces of the support for the multiplicity
  of checksum (and compression) types from apt-rpm (thx imz@):
  + the type of the compression for "pkglist" indices;
  + the type of the checksum for "pkglist" indices;
  + the type of the checksum for "rpm" archives.
- Added blake2b hash support (thx glebfm@).
- Changed file and copy download methods to always compute checksums
  (thx glebfm@).