warn if both `tensorboard` and `tb-nightly` pip packages are installed

This is a common source of inscrutable errors, because pip will happily install both pip packages even though they both ship the tensorboard python package [1], and in doing so the second one will overwrite any files from the first one: https://github.com/pypa/pip/issues/4625. This means you can easily end up with a garbled frankentensorboard package (and it gets even worse if you uninstall one of them but not both of them, since then you get a frankentensorboard with a lot of vital organs removed).

Until pip fixes this mess, it's a fact of life, but we can at least try to warn users about this pitfall ourselves.

[1] It's colloquial to call both of these packages, so I'm prefixing to avoid confusion. See also https://github.com/pypa/packaging.python.org/issues/106

Start by locating TensorBoard's startup or environment-detection entry point and inspect how installed Python packages can be identified. Done means users receive a clear warning when both tensorboard and tb-nightly are installed, without changing behavior otherwise; the issue names no files or tests, so add or locate appropriate coverage during investigation.