Allows use of the new embed feature.

Previously this error was only occuring with `go mod tidy` and was being
ignored. With go 1.16 the error is triggered during `go get` so we need to
handle it again.

Distinguishing the error from GitDependenciesNotReachable also took some
extra effort. The error message has changed from:
github.com/dependabot/vgotest imports
	github.com/googleapis/gnostic/OpenAPIv2: module github.com/googleapis/gnostic@latest found (v0.5.1), but does not contain package github.com/googleapis/gnostic/OpenAPIv2

to:
github.com/dependabot/vgotest imports
        github.com/googleapis/gnostic/OpenAPIv2: cannot find module providing package github.com/googleapis/gnostic/OpenAPIv2

We now use `go list` to check that github.com/googleapis/gnostic is a
reachable repo/module.