Was libbitcoin a part of bitcoin core in the past?
libbitcoin is not, and was never a part of Bitcoin Core. Both are standalone implementations of the Bitcoin protocol.

Bitcoin Core, in its build process, will create a number of files named "libbitcoin_...". These are just locally generated files that are unrelated to the libbitcoin project.

Bitcoin Core does not have any files with the "hpp" extension. So are you sure you're actually trying to build Bitcoin Core?