gdb/dwarf: make some more functions methods of cutu_reader

These are only used by cutu_reader, so make them methods of cutu_reader.
This makes it a bit more obvious in which context this code is called.

lookup_dwo_unit_in_dwp can't be made a method of cutu_reader, as it is
used in another context (lookup_dwp_signatured_type /
lookup_signatured_type), which happens during CU expansion.

Change-Id: Ic62c3119dd6ec198411768323aaf640ed165f51b
Approved-By: Tom Tromey <tom@tromey.com>