We're migrating Blubber to the WMF GitLab instance. For a go project,
this means all package base names need to change for use with go tools.

Replaced all references to gerrit.wikimedia.org/r/blubber with
references to gitlab.wikimedia.org/repos/releng/blubber and ran `go mod
tidy`.

There is documentation around contributing that still needs updating,
but that can be done in a follow up.

Bug: T321225
Change-Id: I6388cbefd796963bf8bcf6e5b7ef84b0142f1dd6