Consolidate reporting result of a dependency check

If successful, we should identify the method which was successful
If successful, we should report the version found (if known)
If failing, we should identify the methods we tried

Some dependency detectors which had no reporting now gain it

There's all kinds of complexities, inconsistencies and special cases hidden
in the existing behaviour, e.g.:

- boost reports modules requested, and BOOST_ROOT (if set)
- gtest/gmock report if they are a prebuilt library or header only
- mpi reports the language
- qt reports modules requested, and the config tool used or tried
- configtool reports the config tool used
- llvm reports if missing modules are optional (one per line)

We add some simple hooks to allow the dependency object to expose the
currently reported information into the consolidated reporting

Note that PkgConfigDependency() takes a silent: keyword which is used
internallly to suppress reporting.  This behaviour isn't needed in
find_external_dependency().

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