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.

include README.rst
include CHANGELOG.rst
include LICENSE.rst
recursive-include piecrust *.py mime.types
recursive-include piecrust/resources *
recursive-include tests *.py
#global-exclude .DS_Store
#global-exclude Thumbs.db
#global-exclude Desktop.ini
#global-exclude *.swp
#global-exclude *~
#global-exclude *.bak