[MM-52397] Move client package into root directory

[hanzei]

Summary

This makes the package more discoverable and makes it clear that it's not part of the plugin built.

Ticket Link

https://mattermost.atlassian.net/browse/MM-52397

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c712474) 15.46% compared to head (7052368) 15.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   15.46%   15.46%           
=======================================
  Files          15       15           
  Lines        5497     5497           
=======================================
  Hits          850      850           
  Misses       4605     4605           
  Partials       42       42           

Impacted Files	Coverage Δ
client/client.go	52.50% <ø> (ø)

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