Add module readmes to json output

[JMBeresford]

Resolves #2499

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This change attempts to fix the above by mimicking the behavior of the ProjectReflection json serialization, which properly outputs the json readme property.

I skimmed through the test suites, but could not find a good place to integrate a test for this at a glance. Let me know which existing tests should be updated (if needed) and where new test logic should be implemented and I'll dig into it, assuming this change in behavior is acceptable.

[Gerrit0]

I'm fine merging this without a test update -- I don't have a good way to test packages mode as thoroughly as I'd like yet.

[Gerrit0]

Looks great, thanks!