use `go get` when VCS method not supported

I noticed error (on https://gowalker.org/src.d10.dev/command):

Oops! Something is going wrong...
src.d10.dev/command: check package: VCS not supported: fossil

My suggestion is, rather than custom support for fossil along with numerous other VCS systems, use go get to fetch this package. go get already understands a wide variety of VCS.

Thanks for the feedback! Might migrate to use go get for all packages when adding support of Go modules.