Delete old files from dist on build

[jvarho]

Since #1029 rebuilding locally keeps accumulating rollup modules, making local builds difficult to deploy.

Each time you run yarn in root or under lambda-at-edge a new set of modules is created. They all get included in any builds that link to the local repository, like e2e-tests.

Not sure this is the best solution, but AFAICT rollup itself doesn't have an option to clean up the build directory.

[codecov]

Codecov Report

Merging #1034 (94f741d) into master (31f3d34) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1034   +/-   ##
=======================================
  Coverage   82.30%   82.30%           
=======================================
  Files          69       69           
  Lines        2577     2577           
  Branches      615      615           
=======================================
  Hits         2121     2121           
  Misses        390      390           
  Partials       66       66           

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[dphang]

LGTM to ensure no extra chunks are generated. Actually I'll add another script to also clean entire dist directory first when calling yarn (e.g for fresh install, rebuilding entire project or publishing). But this is still useful in case you just want to just build the handlers or call rollup manually.