Make --read-only a separate mixin

[Ericson2314]

Motivation

This functionality was part of SourceExprCommand. However, it is actually independent parsing installables or source expressions, except for the fact that parsing installables is one of the many things influenced by read-only mode.

We can make it a mixin instead, which separates concerns.

Context

Cleaning things up so NixOS/rfcs#134 is easy to implement. We will be surgically separating InstallableCommand from SourceExprCommand as part of that, and making those simpler first helps make that task less difficult.

Depends on #7754

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[Ericson2314]

This is similar in spirit to #7754, which also split out a flag to a mixin.

[tomberek]

I'm not clear on whether the defaults are changing for read-only. Is InstallableCommand changing to have a default value to true now?

[thufschmitt]

I'm not clear on whether the defaults are changing for read-only. Is InstallableCommand changing to have a default value to true now?

Nah, it shouldn't. The source of truth is settings.readOnlyMode which is false by default

[Ericson2314]

Thanks @thufschmitt for beating me to the reply :)

@tomberek I should have made clearer in the PR description that supportReadOnlyMode just controls whether the flag exists, and doesn't influence the default value of settings.readOnlyMode in any way.