Since the question mentioned third-party libraries and not all packages then you probably want to fall back to using wildcards.
A use case being: I just want to update all my packages that are obtained from the Github VCS, then you would just say:
go get -u github.com/... // ('...' being the wildcard).
This would go ahead and only update your github packages in the current $GOPATH
Same applies for within a VCS too, say you want to only upgrade all the packages from ogranizaiton A's repo's since as they have released a hotfix you depend on:
go get -u github.com/orgA/...