coredata: Introduce a class to act as the dependency cache

Handling the PKG_CONFIG_PATH variable in meson introduces a new problem
for caching dependencies. We want to encode the pkg_config_path (or
cross_pkg_config_path if we're cross compiling) to be part of the key,
but we don't want to put that into the key for non-pkg-config
dependencies to avoid spurious cache misses (since pkg_config_path isn't
relevant to cmake, for example). However, on a cache lookup we can't
know that a dependency is a pkg-config dependency until we've looked in
the cache.

My solution is a two layer cache, the first layer remains the same as
before, the second layer is a dict-like object that encapsulates the
dependency type information and uses pkg_config_path and
cross_pkg_config_path as a sub key (and could be extended easily for
other types). A new object type is introduced to encapsulate this so
that callers don't need to be aware of the implementation details.

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