cpp-common/bt2: add and use `bt2::internal::Dep*` type alias templates

cpp-common/bt2: add and use `bt2::internal::Dep*` type alias templates

Those new templates take care of using `std::conditional` instead of
having to use it at each site. The template parameters of
`bt2::internal::DepType` are:

1. What to check for constness.
2. The effective type if `LibObjT` is NOT `const`.
3. The effective type if `LibObjT` is `const`.

Other `bt2::internal::Dep*` reuse `DepType` with specific types for
parameters 2 and 3 for common uses.

For example:

    using UserAttributes =
        typename std::conditional<std::is_const<LibObjT>::value,
                                  ConstMapValue,
                                  MapValue>::type;

gets replaced by:

    using UserAttributes = internal::DepUserAttrs<LibObjT>;

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I434b9ae82d170c6fe8359488e78c0e3e34a064cd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11234
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>