Use requirements.txt to lock dependencies

[marstr]

As part of #8592, I updated our dependencies in the azure-cli package to describe their semantic range of support. However, for the sake of actual deployments and test runs, we want to have a locked-down set of dependencies that we use. There are several options for tooling to help with this. However, the first step is to just have something. The simplest way to handle this is to provide requirements.txt based on the different platforms that we're targeting.

[marstr]

#9785 takes care of the first part of this effort, providing the concrete requirements. The second part will be adjusting our installers/test infrastructure to use the appropriate requirements.*.txt file.

[yonzhan]

add to S165.

[fengzhou-msft]

RPM, Deb and Docker already use requirements.*.txt file now. MSI and Homebrew still use setup.py for dependencies. For pip, we can use pip install -r requirements.txt.

I will evaluate the work to change MSI and homebrew.

[fengzhou-msft]

add @haroldrandom to check CI and tests.

[fengzhou-msft]

azdev setup, CI and packaging except pypi are using requirements.txt to lock dependencies now. This issue can be closed.

Documentation on pip install with requirements.txt or modify the linux install script will be another discussion.