Make cpe an allocated property.
Refactor: Extract funciton: cpe_session_new
Refactor: Extract function: cpe_session_free.
Refactor: Extract function: cpe_session_lookup_oval_session.
Use oscap_source to parse cpe_lang_model.

The cpe_validate_xml could be dropped as it does basically nothing.

(XML_PARSE_DTDATTR is useless).
Refactor: Extract function: cpe_lang_model_import_source.
Drop cpe_lang_model_parse_xml.
Introduce: cpe_session_add_cpe_lang_model_source.
Always use oscap_source to import cpe_lang.
Deprecate: cpe_lang_model_import.
Use oscap_source for parsing cpe_dict.

Drop cpe_validate_xml. XML_PARSE_DTDATTR does not imply any validation.
Drop cpe_parser_ctx_new.
Refactor: Extract function: cpe_dict_model_import_source.
Drop cpe_dict_model_parse_xml
Introduce: cpe_session_add_cpe_dict_source.
Always use oscap_source to import cpe_dict.
Refactor: Extract function: cpe_session_add_cpe_autodetect_source.
Move default_cpe initializator to cpe_session.
Introduce xccdf_policy_model_get_cpe_session.
Introduce: ds_sds_session_get_component_sources.