assert rebuildLocalPackages in capture
Add many buildmessage.capture/assertInCapture

Moving towards a world where all things that might invoke buildmessage.error are
encouraged to be in a buildmessage.capture.

This commit is the answer to the question "how many small changes need to be
made to add buildmessage.assertInCapture to PackageCache.loadPackageAtPath?"

Next steps include:
 - Making catalog.resolveConstraints ALWAYS buildmessage.assertInCapture
   (not just when ignoreProjectDeps isn't passed)
 - Then changing resolveConstraints to complain using buildmessage
 - Removing the process.exit(1) in _ensureDepsUpToDate
 - Adding a more structured way to ensure that most commands
   call _ensureDepsUpToDate at an unsurprising location