libdwfl: Handle debugaltlink in dwfl_standard_find_debuginfo.

libdwfl: Handle debugaltlink in dwfl_standard_find_debuginfo.

When we fall back to the debuginfod client then we need to do the
same trick we do for local lookups in dwfl_build_id_find_debuginfo.
If the debug file (dw) is already set, then we must be looking for
the altfile. But we cannot use the actual file/path name given as
hint. We'll have to lookup the alt file "build-id". Because the
debuginfod client only handles build-ids.

Previously we would use the build-id of the main file which meant
the debuginfod client would give us another copy of the debug file,
which would then be set as its own altfile. This caused lots of
confusion...

Signed-off-by: Mark Wielaard <mark@klomp.org>