This builds on #1623 and is another step towards separating cred
computation from plugin declarations, as described in #1557. Basically,
this will allow the frontend to get plugin declarations even if the
TimelineCred computation never saw them.
This commit modifies `api/load`, and adds a new facility to
`DataDirectory` for saving the PluginDeclarations (which will be used by
@Beanow's in-flight refactor of `api/load`).
Test plan: See included unit tests, also try loading a project and
inspect the newlys saved file.
