Fix minor errors in CMake configuration

[vyasr]

This PR fixes a number of small errors that slipped through in #644 and #633. None of them materially affect the build (whether a preexisting libraft exists or not) since 1) benchmarks are currently turned off by default anyway in the C++ build, and 2) pyraft only depends on libraft for headers, not for compiled libraries.

[vyasr]

I don't yet have permissions to add labels, otherwise I would have added DO NOT MERGE instead of marking this as draft.

The reason this issue didn't crop up before is that after getting the UCX/NCCL dependencies finalized in #644 I did not go back and rebuild/test pyraft without a precompiled C++ build. The typos I fixed here will help fix that case, but they aren't sufficient. I will also need to install the built libraft library into the install tree and modify the rpaths for the dask/common and common/ modules. I will make these changes locally and test before we merge.

[cjnolet]

And hopefully by the time this is ready to merge, you'll have the permissions to merge it yourself

[vyasr]

@cjnolet so it turns out that I did in fact previously test this, and the current build will work even without the changes that I proposed in my last message. The important distinction between pyraft and pylibraft is that pyraft actually doesn't depend on raft compiled libraries at all, it just requires some of the headers. Those headers are already being inherited by linking in the raft::raft target, so we should be set. I've done a little more cleanup here, but aside from that I think we're good.

[cjnolet]

Want to go ahead and merge it?

[vyasr]

@gpucibot merge

[vyasr]

Done!