Deprecate: ds_sds_decompose.
Deprecate: ds_sds_decompose_custom.
Deprecate: ds_sds_index_import
Avoid use of xmlReaderForFile

Also avoid use of xmlTextReaderSetErrorHandler and libxml_error_handler.
Drop libxml_error_handler.
tests: Introduce test for a codestyle

Let's forbid ourselves from useing certain functions.
Avoid using xmlTextReaderReadString function.

It requires that we have correct position on the TextNode.
Interestingly, there were no problems with native xmlTextReader,
but with xmlWalkerReader we are hitting xmllib2 TODO like:

    Unimplemented block at xmlreader.c:1794
Tests: Forbid use of xmlTextReaderReadString.
Avoid use of cpe_dict_model_import
Refactor: rename variable.

This needs to be done to improve readability. This whole relative
versus absolute paths is a mess. There are some variables that can
carry only filename others can carry filepath (some relative, some
absolute or some both).

There is a benefit of passing down the relative path. Because relative
path is the catalogue value and the name how the file is known to
other components.
ds_sds_session switches from absolute paths to relative

The firstly it seemed that it will be much more easier to calculate
realpath of each file. However, later on, standard realpath() function
stopped working. As we stopped to create directories and storing files
on the disc.

Problematic were the tests from tests/API/XCCDF/applicability that
use oval component with filepath like: ../unittests/oval.xml. When
the ds_sds_session uses /tmp/tmp.XXXXXX it will attempt to get realpath
of /tmp/unittests and that will fail. Stopping the ds_sds_session
from finding appropriate oscap_source for the given file.
Deprecate: cpe_dict_model_import.