Consistent naming for library-packages

In the [context](https://github.com/conda-forge/arrow-cpp-feedstock/pull/146#issuecomment-634975825) of moving pyarrow into the arrow-cpp-feedstock, there was some discussion of naming the outputs, and I brought up the following:

> Regarding the naming conventions, I think it *might* be worth renaming `arrow-cpp` to `libarrow`. This is in line with a lot of other feedstocks, but obviously a matter of taste. In this case, we'd have to have a compatibility output of `arrow-cpp` that depends on `libarrow`, which could be dropped after a few versions.
>
> @isuruf made me aware of this in the context of [faiss](https://github.com/conda-forge/faiss-split-feedstock/blob/master/recipe/meta.yaml). Other examples I can think of off the top of my head are [blas](https://github.com/conda-forge/blas-feedstock/blob/master/recipe/meta.yaml) & [lapack](https://github.com/conda-forge/lapack-feedstock/blob/master/recipe/meta.yaml), [openblas](https://github.com/conda-forge/openblas-feedstock/blob/master/recipe/meta.yaml), [opencv](https://github.com/conda-forge/opencv-feedstock/blob/master/recipe/meta.yaml), [postgresql](https://github.com/conda-forge/postgresql-feedstock/blob/master/recipe/meta.yaml), [gdal](https://github.com/conda-forge/gdal-feedstock/blob/master/recipe/meta.yaml), plus a [whole bunch](https://github.com/conda-forge?q=lib) more (non-exhaustive).
>
> Of course, this recipe has some counter examples to this: [aws-sdk-cpp](https://github.com/conda-forge/aws-sdk-cpp-feedstock/blob/master/recipe/meta.yaml), [boost-cpp](https://github.com/conda-forge/boost-cpp-feedstock/blob/master/recipe/meta.yaml), [grpc-cpp](https://github.com/conda-forge/grpc-cpp-feedstock/blob/master/recipe/meta.yaml), [thrift-cpp](https://github.com/conda-forge/thrift-cpp-feedstock/blob/master/recipe/meta.yaml), but I think they are far in the [minority](https://anaconda.org/search?q=-cpp) and the same argument could be made for renaming those (maybe worth noting that at least the last two seem to have been started by the arrow team).

Following @isuruf's and @xhochy's input, I'm opening this issue here. Also note @isuruf's comment:
> @isuruf: `-cpp` was a trend that I started with `boost-cpp`. That was a mistake. I'm in favour of changing it, but this is not the correct place to raise the issue.