CMake: Use juce_add_modules to import modules from install tree

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.