[now dev] Add support for deployments without now.json

[AndyBitz]

Add zero config for now dev

Requires this to be on stable: https://github.com/zeit/now-builders/pull/705

[codecov-io]

Codecov Report

Merging #2498 into canary will increase coverage by 0.29%.
The diff coverage is 48.93%.

@@            Coverage Diff             @@
##           canary    #2498      +/-   ##
==========================================
+ Coverage   11.56%   11.86%   +0.29%     
==========================================
  Files         257      257              
  Lines        9258     9306      +48     
  Branches     1031     1045      +14     
==========================================
+ Hits         1071     1104      +33     
- Misses       8074     8083       +9     
- Partials      113      119       +6

Impacted Files	Coverage Δ
src/util/dev/builder.ts	67.85% <ø> (+0.19%)	⬆️
src/util/get-files.ts	90.18% <ø> (+0.31%)	⬆️
src/util/dev/server.ts	51.11% <48.93%> (+1.44%)	⬆️
src/util/metrics.ts	61.53% <0%> (-6.65%)	⬇️
src/util/dev/builder-cache.ts	63.33% <0%> (-1.33%)	⬇️
src/util/output/create-output.ts	40.54% <0%> (+2.7%)	⬆️

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[TooTallNate]

Could we add some unit tests to the test/dev-server.unit.js file with some fixtures utilizing this new system?