[now dev] Fix index-based asset lookup logic

[TooTallNate]

There was a bug in the index lookup logic such that GET /404 would be matching the /index.* file. Fixed in this PR.

[codecov-io]

Codecov Report

Merging #2138 into canary will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           canary   #2138      +/-   ##
=========================================
- Coverage    4.44%   4.43%   -0.01%     
=========================================
  Files         243     243              
  Lines        8648    8652       +4     
  Branches      901     902       +1     
=========================================
  Hits          384     384              
- Misses       8247    8251       +4     
  Partials       17      17

Impacted Files	Coverage Δ
src/commands/dev/lib/dev-server.ts	0% <ø> (ø)	⬆️

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