puffin-resolver: simplify version map construction (#1267)
In the process of making VersionMap construction lazy, I realized this
refactoring would be useful to me. It also simplifies a fair bit of case
analysis and does fewer BTreeMap lookups during construction. With that
said, this doesn't seem to matter for perf:
```
$ hyperfine -w10 --runs 50 \
"puffin-main pip compile --cache-dir ~/astral/tmp/cache-main ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null" \
"puffin-test pip compile --cache-dir ~/astral/tmp/cache-test ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null"
Benchmark 1: puffin-main pip compile --cache-dir ~/astral/tmp/cache-main ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null
Time (mean ± σ): 146.8 ms ± 4.1 ms [User: 350.1 ms, System: 314.2 ms]
Range (min … max): 140.7 ms … 158.0 ms 50 runs
Benchmark 2: puffin-test pip compile --cache-dir ~/astral/tmp/cache-test ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null
Time (mean ± σ): 146.8 ms ± 4.5 ms [User: 359.8 ms, System: 308.3 ms]
Range (min … max): 138.2 ms … 160.1 ms 50 runs
Summary
puffin-main pip compile --cache-dir ~/astral/tmp/cache-main ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null ran
1.00 ± 0.04 times faster than puffin-test pip compile --cache-dir ~/astral/tmp/cache-test ~/astral/tmp/reqs/home-assistant-reduced.in -o /dev/null
```
But the simplification is still nice, and will decrease the delta
between what we have now and a lazy version map.
