Support installations with swap files

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Summary

On modern systems, it may be desirable to use a swap file instead of a separate swap partition. There are a few benefits of this:

1. A swap file can be placed on any mounted file system
2. A swap file can be set to any desired size
3. Swap files are more space efficient

Users on older hardware may still have a better experience with a separate swap partition, so support for both should be kept, assuming we decide to support files. The Calamares installer can automatically create a swap file during installation if enabled for the swap dropdown menu, and it seems to Just Work ™️ in a VM using BTRFS. Of course, that's only one possible configuration. It would also have the downside of increasing our release testing matrix.

Steps to reproduce

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Expected result

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Actual result

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Environment

• Is system up to date?

Repo

Unstable

Desktop Environment

Budgie

System details

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Other comments

This is the closest template we have for a feature request.

No files, tests, or entry points are identified in the issue. First locate the installation and swap-selection code, then clarify the supported swap-file configurations and testing scope; done means installations can offer swap files while retaining swap-partition support.