Custom installed dependency resolving

[mwriter]

What's the problem this feature will solve?

There is no equivalent option --target to search for installed dependencies in the specified directories to skip their installation. Or rather, equivalent of python sys.path for pip. Although by default pip searches in python site-packages directory and skips the installation of already installed packages.

Describe the solution you'd like

It seems to me that there should be some option to search for installed dependencies in the target directory itself. But it is much better if can specify any number of search paths.

Alternative Solutions

Independently build the entire dependency tree, determine which dependencies are already installed and install the missing ones.

Additional context

https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-t

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