Flattening Imperative and separating out tests

[ATorrise]

What It Does

• Flattens zowe-cli/packages/imperative
  • takes tests out of src
  • removes redundant file structure ie src/packageName/src
  • an effort towards standardizing imperative's structure to that of other packages' structure
• Allows 1 ts-configuration file to apply over entire zowe-cli/packages/imperative/__tests__
• Is part of the clean-up effort for v3

Review Checklist
I certify that I have:

• tested my changes
• added/updated automated tests
• updated the changelog
• followed the contribution guidelines

Additional Comments

• Unsure about how to incorporate changelog
• fixing up imperative package inspired by previous tsconfig-related PR

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (841d840) 91.03% compared to head (e5b1edb) 76.27%.
Report is 248 commits behind head on next.

Files	Patch %	Lines
.../cli/src/workflows/create/Create.common.handler.ts	0.00%	5 Missing ⚠️
...rkflows/start/workflowStep/WorkflowStep.handler.ts	0.00%	2 Missing ⚠️
...veWorkflowDetails/ActiveWorkflowDetails.handler.ts	0.00%	1 Missing ⚠️
...ws/list/activeWorkflows/ActiveWorkflows.handler.ts	50.00%	1 Missing ⚠️
...ist/archivedWorkflows/ArchivedWorkflows.handler.ts	50.00%	1 Missing ⚠️
...owDefinition/RetrieveWorkflowDefinition.handler.ts	50.00%	1 Missing ⚠️
...rkflows/start/workflowFull/WorkflowFull.handler.ts	0.00%	1 Missing ⚠️
packages/core/src/rest/ZosmfRestClient.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             next    #2020       +/-   ##
===========================================
- Coverage   91.03%   76.27%   -14.76%     
===========================================
  Files         635      635               
  Lines       18610    18629       +19     
  Branches     3859     3866        +7     
===========================================
- Hits        16941    14209     -2732     
- Misses       1668     4418     +2750     
- Partials        1        2        +1     

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