We install the protos used in external/googleapis because if you install the C++ libraries maybe you want to use the installed protos in your own proto files.

Now, they used to get installed in $PREFIX/include/..., same path as the headers, but now they are getting installed in a weird directory like $PREFIX/include/src/googleapis_download/... that seems terrible.

Evidently we do not have a test for these, so we need to fix that too.