Look for azcopy.exe in AZCOPY if it's a directory

[kananb]

Summary of the Pull Request

#3234

PR Checklist

• Applies to work item: #xxx
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

Adds code to look for azcopy.exe inside of AZCOPY if it's a directory and prints a nice error if azcopy.exe can't be found.

Validation Steps Performed

Set AZCOPY to a directory containing azcopy.exe and observe that commands work as expected.
Set AZCOPY to a directory not containing azcopy.exe and observe that an informative error is printed out.
Set AZCOPY to a path pointing to azcopy.exe and observe that commands work as expected.
Set AZCOPY to a path pointing to an azcopy.exe that doesn't exist and observe that an informative error is printed out.

[codecov-commenter]

Codecov Report

Merging #3344 (e0c100b) into main (6ea2faa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3344   +/-   ##
=======================================
  Coverage   31.66%   31.66%           
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  Files         308      308           
  Lines       37609    37609           
=======================================
  Hits        11909    11909           
  Misses      25700    25700