Use homeModules as flake output name for Home Manager modules (backport #3387)

[mergify]

This PR contains a handful of changes centered around the flake output for Home Manager modules. My understanding is that, recently, convention has been converging on using homeModules as the name of the flake output (as this is closer to homeConfigurations and more in line with nixosModules/nixosConfigurations), which is the focus of this PR.

This PR contains a few commits. The first simply adds homeModules as an alias of homeManagerModules. The second migrates documentation and internal uses of homeManagerModules to use homeModules instead, and the third inverts the first by defining homeManagerModules in terms of homeModules, treating it as a backward-compatibility layer. I'm not sure what the stance of the project is on this issue, but while I'd personally like to see all three land, just having homeModules available as an option (i.e., just the first commit here) would satisfy me.

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This is an automatic backport of pull request #3387 done by Mergify.

[mergify]

This pull request, with head sha cfea16cdbe4f13b5d39dfe3df747092448252c9d, has been successfully merged with fast-forward by Mergify.

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