Installed package detection in requirement.txt

[karthiknadig]

Refs: #21140

• windows @devinvalenciano
• linux @connor4312
• macOS @sandy081

Complexity: 2

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Requirements

1. Python 3.7 or greater
2. Latest python extension pre-release.

Verification steps:

1. Ensure you have python and python extension installed.
2. Make sure python extension is activated (open python file)
3. Create requirements.txt with following content:

flake8
flake8-docstrings; os_name != 'nt'
httpx

Things to check:

1. For os specific case you should see the create env button only if the dependency that applies to your os (as in the constraint passes). More environment markers here (https://peps.python.org/pep-0508/#environment-markers)
2. If there are no dependencies or if all packages are installed then Create Environment button should go away.
3. You should be able to see some hint squiggle for cases where the package is missing.

Note: we don't trigger on file system events to detect package installs at this point. So you will need to close and reopen the file for this.
Note 2: We don't validate versions at this point. We only detect if a package is installed.