[now dev] Fix directory listing

[amio]

Treat a project as static deployment if now.json match one of these:

• no builds
• all builds have @now/static as use

[leo]

Works!

BEFORE

AFTER

[codecov-io]

Codecov Report

Merging #2256 into canary will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           canary    #2256      +/-   ##
==========================================
+ Coverage   10.43%   10.45%   +0.01%     
==========================================
  Files         246      246              
  Lines        9024     9040      +16     
  Branches      990      994       +4     
==========================================
+ Hits          942      945       +3     
- Misses       7958     7968      +10     
- Partials      124      127       +3

Impacted Files	Coverage Δ
src/commands/dev/lib/dev-server.ts	38.82% <100%> (-0.79%)	⬇️

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