Build stalls on stalled download instead of scheduling other tasks

I often happen to work behind a very unreliable and slow internet connection which often results in stalled package downloads. Here's a snippet from one such occasion:

```
$ stack build
...
conduit-extra-1.1.13.1: download
adjunctions-4.3: build
adjunctions-4.3: copy/register
vector-algorithms-0.7.0.1: copy/register
PrProgress: 122/139user interrupt
$ stack build
keys-3.11: configure
keys-3.11: build
kan-extensions-4.2.3: configure
kan-extensions-4.2.3: build
either-4.4.1: configure
conduit-extra-1.1.13.1: download
...
```

So the download of `conduit-extra` stalled and after `vector-algorithms` had been registered nothing happened for quite a while until I pressed Ctrl-C.

What surprised me, was that when I restarted the build, three packages immediately started to build before the download restarted.

Why weren't these packages already built while the download was stalled? Is this possibly an issue with the build plan?