Consider putting plugins in a different location

[SwooshyCueb]

At present, all the plugins live in the same directory as the irods_s3_api binary. We should consider changing how plugins are loaded so that they can live in a separate directory. This will allow us to put plugins somewhere like ${LIBDIR}/irods/clients/s3_api and the main executable in ${BINDIR}.

[JustinKyleJames]

Just a note: I temporarily removed the plugin architecture from the project while we are transitioning away from coroutines. This will be restored later.