Refactor: Extract function: rds_index_import_source
Make a use of rds_index_import_source

Each of the changes avoids an extra file open/parse.
Deprecate function: rds_index_import
Promote rds_index_parse from static to privileged.
Introduce: ds_rds_session_get_rds_idx
Make a use of ds_rds_session_get_rds_idx
Make a use of ds_rds_session_get_rds_idx
Drop rds_index_import_source.

Introducing was not a good move. It will be better to keep rds_index
initialized (parsed) only by ds_rds_session. That approach has following
two benefits: (1) ds_rds_session operations will mimic the
ds_sds_session, (2) callers will not be tempted to parse the thing
twice. The initialization of ds_rds_session in oscap-info only for sake
of getting index is cost I prefer to undertake.
Use oscap_source to save arf components (arf:content)
tests: The oscap_source capitalized encoding.
Do not guess request_id when splitting ARF.

This is fishy. If there is no report-request in relationship with the
report we will try to find the very first one. This seems incorrect,
because we were requested to dump a couple of specific IDs. (compare
code of utils/oscap-ds.c with rds.c).
Refactor: Extract function: _lookup_in_arf
Refactor: Make ds_rds_dump_arf_content return oscap_source

This actually grows the size of code a little bit. That's because we
are adding error handling that has been previously missing.
Introduce internal cache to ds_rds_session.

This follows the very same patter as ds_sds_session.
Promote mkdir_p() to oscap_acquire module.

It is good fit there. Then it is already used outside of DS module
and we can get a rid of MAX_PATHLEN this way.
Refactor: promote oscap_acquire_ensure_parent_dir
Introduce: ds_rds_session_dump_component_files

This follows the very same patter as ds_sds_session.
Use ds_rds_session to save ARF components.
Introduce ds_rds_session_get_xmlDoc

This follows the very same patter as ds_sds_session.
Refactor: Pass rds_session downto ds_rds_dump_arf_content.