Was libbitcoin a part of bitcoin core in the past? I am trying to use c++ code from the book of Antonopoulos for some time now and I seem to lack an include named bitcoin/bitcoin.hpp.

To compile the code the g++ compiler gets a pkg-config input with libbitcoin. So I suspect I would have to install this library. 

Since the book does not mention to install anything else then bitcoin core I really wonder if parts of libbitcoin have originally been part of bitcoin core and have been excluded at some point. 

The other explanation would be that the author forgot to mention it. But it seems a bit overload that I have to install another complete system for a full node (libbitcoin) to be able to run the code since the book explicitly says that running a full node with bitcoin core should suffice. 

It is just a minor thing and probably laughable for an expert in the field but I am also learning how to use pkg-config and the compiler by doing this. 

There have been suggestions to just install parts of libbitcoin but the developers said it is advisable to install the full thing with the automatic install skript. 

Also, there is libbitcoin-dev, which is a package in debian (I run Raspbian Buster), but I was not able to compile the code with it.
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?