pkg: introduce -f|--force for pkg install

[efiop]

Fixes #1773

Signed-off-by: Ruslan Kuprieiev ruslan@iterative.ai

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• Does your PR affect documented changes or does it add new functionality
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iterative/dvc.org#385

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[shcheklein]

@efiop could describe what kind of use case do you have in mind?

[efiop]

@shcheklein Mostly for the cases when you have a corrupted installation of some package. Also useful if you have installed a package say, from your own fork at https://github.com/efiop/dvc and now want to replace it with the upstream version https://githug.com/iterative/dvc. Also you could use it to update/upgrade, but that usecase will be handled by dvc pkg update in #1774).