It's helpful if everyone using the package can use the same name to refer to its contents, which implies that the package name should be good: short, concise, evocative. By convention, packages are given lower case, single-word names; there should be no need for underscores or mixedCaps.
http://golang.org/doc/effective_go.html#package-names
It's mostly a problem of taste, but changing the project name into mapset or even changing both the project name and the package name into something like goset or something else would be wonderfully nice. Because it's simply recommend by the Go authors. :D