Fix unmock-server manifest.

[ksaaskil]

• Running npm i -g unmock-server followed by unmock-server start fails with the following: Error Plugin: unmock-server: Cannot find module 'globby'. This discussion suggests that the problem is missing oclif.manifest.json, so add a prepack step that adds the missing file with manifest command of oclif-dev CLI.
• Also change commands to point to dist/commands, it should probably point to the compiled version instead of TS source

[codecov-io]

Codecov Report

Merging #371 into dev will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #371   +/-   ##
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  Coverage   80.35%   80.35%           
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  Files          54       54           
  Lines        2428     2428           
  Branches      585      585           
=======================================
  Hits         1951     1951           
  Misses        477      477

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