Add --no-sync option to pdm-update

[blueyed]

When updating requirements (e.g. via pdm update --update-eager) it should not automatically sync/install dependencies.

(My use case here is using pdm for managing dependencies, but would like to install them only via exported requirements.txt files then.)

Note that pdm update --update-eager --dry-run looks like the following, creating a __pypackages__ skeleton:

✔ 🔒 Lock successful
Packages to add:
…
% find __pypackages__
__pypackages__
__pypackages__/3.9
__pypackages__/3.9/lib
__pypackages__/3.9/include
__pypackages__/3.9/bin
__pypackages__/.gitignore

--no-sync exists with pdm-add already, and appears to make sense here then also.

[frostming]

What is the problem of --dry-run? Do you want to update the lock file but not install them?

[blueyed]

Do you want to update the lock file but not install them?

Yes.