dependencies: tighten type checking and fix cmake API violation for get_variable

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.

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