Support `pdm outdated`

Is your feature request related to a problem? Please describe.

There is no any related problem.

Describe the solution you’d like

1. command pdm outdated, or pdm list --outdated.
2. pdm prints all the dependencies (in local/global) which is outdated.

Reason: It’s common feature of package manager and I used them often. If this proposal is positive, I will try implementing and send PR.

Remarks: I think pdm outdated is better than pdm list --outdated because I want an exact one option.

• pdm outdated -> returns only top-level dependencies.
• pdm outdated -a(--all) -> returns all dependencies.

Motivation came from this issue https://github.com/python-poetry/poetry/issues/2684#issuecomment-767046631.

[frostming]

Done, now you can test on the master branch. Note that --outdated is just an alias of --dry-run

[frostming]

The design will be as the following:

• add --dry-run to update command to display (to install, to update, to remove) packages but not to modify the pdm.lock.
• Under the hood, lock will be performed to fetch the latest available versions, then show the diff between the result and the installed versions in the current environment
• Other arguments and options such as --dev, sections, packages, --unconstrained and --update-strategy will all influence the result.
• Based on the above, add an option --top to limit the result to top-level packages only(those list in pyproject.toml), again, which can be further limited by --dev, sections. Plus, when --top is passed, specifying packages via packages is not allowed.

If anything is not clear, feel free to reply in this thread