make os.root.baseName equal to s"${os.root/}" rather than throwing an exception

[philwalk]

This addresses the problems described by #276
It provides a fix for VirtusLab/scala-cli#2954
It makes os.root.last the empty string rather than throwing LastOnEmptyPath.
Then, it defines os.root.baseName as s"$driveRoot/", rather than throwing an Exception.

[lihaoyi]

TBH I'm not sure if this is the correct behavior for this. I think it's reasonable for it to fail and downstream to handle it, v.s. arbitrarily returning empty strings.

There are tons of cases where empty strings are not what you want either, and they have a tendency to propagate far beyond the point of creation before misbehaving in complicated ways. In comparison, the exception immediately tells you what's going on and you can remediate it at the failure site

[philwalk]

TBH I'm not sure if this is the correct behavior for this. I think it's reasonable for it to fail and downstream to handle it, v.s. arbitrarily returning empty strings.

I don't disagree with your reasoning. A concern is that throwing of an exception makes it unsafe to call .baseName on any os.Path that might be === os.root. Empty string is a reasonable way to represent that a path has no segments, implying that it's os.root. This is similar to the way that dirname / returns / rather than an error message.

On the other hand, this change might cause problems with existing client code, so perhaps this needs to be handled the way you suggest, thanks for the feedback.