packrat needs repository record in DESCRIPTION if a local CRAN-like repository is used

During packrat:initial(), packrat 0.5.0 tries to infer where to find a package's source, and does so in a large if-then-else construction. If you have a package that has been successfully installed from a CRAN-like repository, but does not actually contain a "repository:" record, then internally the source gets marked as "unknown" and packrat::initial() fails when it does a snapshot. This dependency on a repository record in DESCRIPTION doesn't seem to be documented. At the very least it would be nice to make the error message more helpful by indicating this dependency. But an even better solution might be to have packrat check each configured repositories to see if any hold the source (our "repos" option was correctly pointing to our CRAN-like repository so it would have found it had it checked).

Start at packrat:initial() and trace the source-inference logic used before snapshotting. Reproduce the failure with a package installed from a local CRAN-like repository whose DESCRIPTION lacks a repository record, while the repos option points to that repository. Done means the dependency is handled or the error clearly explains the repository-record requirement.