Re: [boost] bcp namespace aliasing broken, impacts package providers, shared libraries unusable since 1.69

Apart from that, https://travis-ci.org/boostorg/bcp/builds/537447552

After fixing the two problems in Jamroot, testing the library after --namespace still doesn't work, because libs/program_options/test/Jamfile.v2 refers to the library as boost_program_options, but it's no longer called that.