Use a single queue to install dependencies in installDeps

[schomatis]

Increasing ratelim (e.g., to 20 goroutines working simultaneously) seems to improve the download speed (considerably, but more tests are needed to state that with some certainty) up until the first ~100 packages (direct dependencies of go-ipfs) but then it slows down again to download speeds similar to the default ratelim of 2 scenario, when new levels of dependencies are processed in a different call of installDeps which normally has a small number of child dependencies and the parallel fetch is not leveraged.

[schomatis]

This will need to split installDeps into the fetch and the post-install hook process, as the current recursive implementation traversed the dependency DAG in DFS order to run the gx-go doRewrite function (through the hook) from bottom to top (because doRewrite needs the dependencies of a package already available). Since the network fetch seems like the most expensive operation (in contrast with the local rewrite/disk-install) delaying the second process to unify the first one wouldn't defeat the performance gain mentioned above.

[schomatis]

Also, this separation would allow to parallelize the packages rewrite process which is now happening in DFS sequence (although I need to check the doRewrite logic to see if that parallelism is supported).

[Stebalien]

we currently parallelize rewriting files within a package but yeah, rewriting multiple packages at the same time would be nice.

[schomatis]

Actually testing a recent version of gx-go with your commit improving the rewrite logic already makes a huge performance improvement, so more parallelization there won't help much, the fetching part seems like the only bottleneck now.