The CoreAudioKit (and on macOS, AudioUnit) frameworks are required to
host AudioUnit plugins. Hosts (especially those which don't use the
`juce_audio_utils` module) should use the new `PLUGINHOST_AU` parameter
to `juce_add_*` in order to add the correct preprocessor definition and
link the necessary frameworks.

If git/zip-extraction add hidden directories containing files with a
`.h` extension we don't want to try to build those files as PIPs. Using
GLOB instead of GLOB_RECURSE should help to avoid files in nested
directories.

This change means that imported juce modules will be made available both
with and without a namespace prefix, e.g. `juce_core` and
`juce::juce_core` will both be created.

This change allows custom modules to specify dependencies without a
juce:: prefix, which allows the modules to be used with the Projucer, or
under CMake with JUCE in a subdirectory, or under CMake with JUCE
installed to the system.

This change helps to avoid ambiguity in includes when building projects
which contain more than one "BinaryData.h".

This change adds the examples from `examples/Plugins` to the
AudioPluginHost, surfacing them as 'internal' plugins in the popup menu.

We also build this into the AudioPluginHost, as a convenient way to
check the midi outputs of plugins and devices.