cmake: Build generator files in build directory

Treat the source directory as immutable.  Copy the generator directory
which previously generated files in-tree to the build directory and
then generate files.

Many emerging continuous integration build systems test builds across
multiple versions of dependencies protobuf and python versions in
particular.  The previous source tree builds resulted in stale files
from the last build breaking the current build.  By placing the build
files in the build directory, the build system automatically removes
stale files (removes output build directory) and regenerates them as
necessary.

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