sources: Pass any current mode to `_populateMetadata` when finding pages. Page sources like the `prose` source may need to open an existing page's file to read stuff from it. This won't work if the metadata is populated as part of finding a path to create a page (like when running `chef prepare`). We pass the mode to `_populateMetadata` so the underlying class now knows the current context in which it is called.

syntax: glob
*.pyc
venv
tags
build/lib
build/messages/_cache
build/messages/_counter
dist
piecrust.egg-info
piecrust/__version__.py