fix piupartslib imports

Nis Martensenrequested to merge
nma/piuparts:fix_piupartslib_imports into develop

During the port to python3 piupartslib was changed to no longer auto-import some of its submodules. Import the required submodules directly where now needed.

Fixes:

Traceback (most recent call last):
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1910, in <module>
    main()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1849, in main
    section = Section(section_name, master_directory, doc_root, packagedb_cache=packagedb_cache)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 827, in __init__
    self._load_package_database(section, master_directory)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 859, in _load_package_database
    db = piupartslib.packagesdb.PackagesDB(prefix=sectiondir)
AttributeError: module 'piupartslib' has no attribute 'packagesdb'

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