Support for --contstraint file

[prudnikov]

Is your feature/enhancement proposal related to a problem? Please describe.

PIP allows the use of constraint files. You can find more about it here https://pip.pypa.io/en/stable/user_guide/#constraints-files
Apache Airflow is one of the projects that heavily relies on the constraints file. We are having many problems with not being able to use the constraint file. We have to export dependencies to requirements.txt, add this constraint file location, and use PIP to install dependencies instead of using pdm install and leveraging pdm.lock.

I know PDM has a similar feature, but it is not usable with Apache Airflow. Here is an example of it https://raw.githubusercontent.com/apache/airflow/constraints-2.8.1/constraints-3.11.txt. As you can see it is over 700 lines, moving all of them to the pyproject.toml is a huge mess.

Another advantage of being able to specify constraint file is being able to host this file at some centralized location like Apache Airflow does.

It should be nice if PDM supports it. This is a blocker for us and I believe for many teams.

Describe the solution you'd like

Similar to resolution overrides, I can think of something like this

[tool.pdm.resolution]
constraint-file = "https://raw.githubusercontent.com/apache/airflow/constraints-2.8.1/constraints-3.11.txt"

[huxuan]

pdm also supports reuse-installed update strategy. So if you just want to lock the versions according to the constraints, A potential workaround may be installing them before running pdm lock with --update-reuse-installed.