docs: clarify and include licenses of dependencies

While our contribution guide tries to make clear the licenses that apply
to libgit2, it does not make clear that different licenses apply to our
bundled dependencies. Make this clear by listing each dependency
together with the licenses that they are governed by. Furthermore,
bundle the complete license texts next to the code they apply to.

Both the USE_SANITIZER and USE_COVERAGE options are convenience options
that turn on a set of CFLAGS. Despite our own set of CFLAGS required to
build libgit2, we have no real business to mess with them, though, as
they can easily be passed in by the user via specifying the CFLAGS
environment variable. The reasoning behind not providing them is that as
soon as we start adding those for some usecases, users might ask for
other sets of CFLAGS catering to their specific need in another usecase.
Thus, we do not want to support them here.

Right now, users are being instrucded to add the
"-DCMAKE_EXE_LINKER_FLAGS=-fsanitize=fuzzer" flag when they want to
build our fuzzers. This is error-prone and user unfriendly. Instead,
just add the flag to our fuzzers' build instructions so that it happens
automatically. Adjust the README accordingly.