pkg_rpm does not know how to install non-treeartifact directories

E.g. passing something like this:

```starlark
pkg_files(
name = "foo",
srcs = ["this/is/a/directory"],
prefix = ["foo"],
)
```

Causes `rpmbuild` to fail like this (not exactly the error, but close enough):

```
cp: omitting directory 'this/is/a/directory'
D: waitpid(24) rc 24 status 100
D: /bin/sh failed: 100
error: Bad exit status from /tmp/tmpgxeo4xlc/TMP/rpm-tmp.050qqD (%install)
```

Users shouldn't be doing this anyway (and using `glob`s), but anyway, this behavior should be understood and likely documented.