Allow setting alternate default paths

[noamross]

While I appreciate the discipline of only declaring dependencies in ./packages.R, there are conditions that it makes sense to find them across ./R/*.R or other files, and in a project it's best to be consistent across this. This PR allows users to set a default path for all dep_source_path arguments using either options(capsule.sources = <PATHS>) or a CAPSULE_SOURCES environment variable. We would set this in a project-level .Rprofile so capshot(), create(), etc., behave the same for everyone by default.