build-pkg: add second pass

The second pass is done after the first one. During the second pass,
an item is built in tree of files from all other items. It checks that:

  * packages can be built in any order satisfying dependencies;
  * a package can be rebuilt after its dependee.

For both cases, not only build status is checked but also two sets of
files are compared. Currently content of files is not checked, only
their existance in both passes.

See #1111

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