Refactor virtualenv generators

[jgsogo]

Objectives:

• Decouple the script to apply the variables from the file declaring them
  • the script should be exactly the same for any environment (only dependant on the cmd platform: ps1, sh, cmd)
  • all the variables should be contained into a .env file (some usage examples: docker, npm dotenv, heroku,...).
• It should help to simplify the code

It is also needed for the plugins, they need to read environment variables from some file in order to apply them before calling some Conan commands or CMake itself.

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Other things to consider (not the first iteration):

• Should scripts be generated or contained in the Conan codebase (and updated with new versions)? It would allow activation like:
  • $> conan_env <path/to/file.env>, is it possible to use a Conan command for it? Can be created a stack of activated virtualenvs and help the user with it?
  • $> my_local.sh, it will be implemented like <path/to/conan/activate.sh> <path/to/local/file.env>

[jgsogo]

Testing approach suggested here by @ytimenkov should probably be considered first: #4291

[memsharded]

Yes, I agree, that has been a long standing open issue, we couldn't move forward it properly (sorry @ytimenkov), if we are doing this, I agree that recovering that effort for better testing the environments would be a great thing.

[jgsogo]

We've merged the tests (#5828), here I have a POC about using .env file to store the environment variables (https://github.com/conan-io/conan/compare/develop...jgsogo:feat/environment-vars?expand=1). It could be the next step for this issue (needed from the plugins perspective) or we could prioritize some refactor in the generators code to simplify it.