Refactor package resolution logic out of cmd/{worker,analysis}/main.go

[maxfisher-g]

Refactors duplicate logic from cmd/{worker,analysis}/main.go, with slight behaviour changes:

1. The analysis version panicked if both a local package and an explicit version were specified on the command line. According to @calebbrown this behaviour is not necessary and can be removed
2. If manager.Latest returns a non-nil error, the log message changes from Failed to get latest version to Error resolving package and the log tags contain a key "error": "failed to get latest version for package <package name>: <returned error info>"

Are these acceptable?