Resolve Git dependencies offline if tarball is available. (#3000)
Resolve Git dependencies offline if tarball is available. (#3000)

When running yarn offline, Git dependencies get trapped in
hasHTTPCapabiity even though resolveOverHTTP defaults to returning
the tarball. This change allows us to bypass that step altogether
if the tarball already exists.
andreypopp/esy,andreypopp/esy,andreypopp/esy