[library usage] Move modules under pkg to pkg/internal

[bcho]

This pull request is part of the changes for #373

In this pull request we moved most of the sub packages under pkg to pkg/internal except the pkg/cmd. The pkg/cmd is needed to be exported from pkg level as Go's internal package rule limit the internal package visibility level to pkg only. We also hid sub-commands from pkg/cmd to avoid unexpected references.

Now, the root level main function imports only pkg/cmd and it will be served as the sole entry point. This is slightly off from the design doc, but I believe the usage experience would be better as we allow user to install kubelogin via this:

$ go install github.com/Azure/kubelogin@{version}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3e83c88) 64.77% compared to head (659b526) 64.77%.

❗ Current head 659b526 differs from pull request most recent head bffc8a9. Consider uploading reports for the commit bffc8a9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #376   +/-   ##
=======================================
  Coverage   64.77%   64.77%           
=======================================
  Files          23       23           
  Lines        1766     1766           
=======================================
  Hits         1144     1144           
  Misses        549      549           
  Partials       73       73           

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[weinong]

LGTM