[#1220] Implement standalone which command.

Problem description
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`which` brink command does not check if file exists on Windows platforms.

We need this in order to be able to run the tasks regardless if the OS is 32 or 64 bit: location of `Program Files` folder differs.

Changes description
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Add check for file existence and unit/system tests.

How to try and test the changes
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reviewers @adiroiban

Check that the code makes sense and peruse the tests code: I feel there is room for improvement there.

Thanks!