dep.get_variable() only supports string values for pkg-config and
config-tool, because those interfaces use text communication, and
internal variables (from declare_dependency) operate the same way.
CMake had an oddity, where get_variable doesn't document that it allows
list values but apparently it miiiiiight work? Actually getting that
kind of result would be dangerously inconsistent though. Also, CMake
does not support lists so it's a lie. Strings that are *treated* as
lists with `;` splitting don't count...
We could do two things here:
- raise an error
- treat it as a string and return a string
It's not clear what the use case of get_variable() on a maybe-list is,
and should probably be a hard error. But that's controversial, so
instead we just return the original `;`-delimited string. It is probably
the wrong thing, but users are welcome to cope with that somehow on
their own.
Both of these are artifacts of the time before Dependency Factories,
when a dependency that could be discovered multiple ways did ugly stuff
like finding a specific dependency, then replacing it's own attributes
with that dependency's attributes. We don't have cases of that left in
the tree, so let's get rid of this code too
This allow mypy to catch cases where we accidently assign the dependency
name to the type_name, as it sees them as having different types (though
at runtime they're all strings).