Prefetch direct dependencies during init

[carolynvs]

The initial caching of the repositories can be slow, and doesn't print any output because it's happening as a side-effect of the other init logic. We used to explicitly prefetch the repositories and log a message, but that was lost at some point over the past months.

https://github.com/golang/dep/pull/194/files#diff-b14a2eb16f1efbc6aa9f1ffdad724265R241

Let's start doing this again during init (most likely in getDirectDependencies), and cap the concurrency at 4.

Original comment that inspired this issue: #1129 (comment).

[sdboyer]

note that an impl of this should almost certainly use golang.org/x/sync/errgroup, which we'll be introducing in #1155

[xmattstrongx]

@carolynvs I can pick this up. I worked on the original log message story after all 😄