Move UsersLookupByEmailResponse to the right package (source code compatible)

[seratch]

Summary

While working on #484, I've noticed that UsersLookupByEmailResponse is mistakenly replaced under channels package 🤦

This pull request corrects this issue without bringing any breaking changes to existing apps. We'll be removing the deprecated class under channels package since v1.1 release.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #486 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #486   +/-   ##
=========================================
  Coverage     83.80%   83.80%           
- Complexity     2413     2414    +1     
=========================================
  Files           251      252    +1     
  Lines          6482     6483    +1     
  Branches        585      585           
=========================================
+ Hits           5432     5433    +1     
  Misses          687      687           
  Partials        363      363           

Impacted Files	Coverage Δ	Complexity Δ
...slack/api/methods/impl/AsyncMethodsClientImpl.java	98.37% <ø> (ø)	294.00 <0.00> (ø)
.../com/slack/api/methods/impl/MethodsClientImpl.java	92.30% <ø> (ø)	417.00 <0.00> (ø)
...ods/response/users/UsersLookupByEmailResponse.java	100.00% <100.00%> (ø)	1.00 <1.00> (?)

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