Previously, this was failing as follows:
```
these derivations will be built:
/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv
building '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv'...
building
warning: GOPATH set to GOROOT (/nix/store/4859cp1v7zqcqh43jkqsayl4wrz3g6hp-go-1.13.4/share/go) has no effect
failed to initialize build cache at /homeless-shelter/.cache/go-build: mkdir /homeless-shelter: permission denied
builder for '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv' failed with exit code 1
error: build of '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv' failed
```
This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.
This is the first step towards factoring out CLI-based functionality
for Nixery.
Adds an implementation of popcount that, instead of realising
derivations locally, just queries the cache's narinfo files.
The downside of this is that calculating popularity for arbitrary Nix
package sets is not possible with this implementation. The upside is
that calculating the popularity for an entire Nix channel can now be
done in ~10 seconds[0].
This fixes#65.
[0]: Assuming a /fast/ internet connection.
