Move build and list subcommands as part of language-pack command

[grappler]

Description See #155

How has this been tested? Ran wp traduttore language-pack build traduttore-test-project and p traduttore language-pack list traduttore-test-project locally and they worked.

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #156 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #156   +/-   ##
=========================================
  Coverage     79.79%   79.79%           
  Complexity      379      379           
=========================================
  Files            24       24           
  Lines           896      896           
=========================================
  Hits            715      715           
  Misses          181      181

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