[catalog-api] Consider new names for "Readable" and "Unreadable"

[justinfagnani]

Right now we have interfaces ReadablePackageInfo/UnreadablePackageInfo, and ReadablePackageVersion/UnreadablePackageVersion to represent the difference between documents that have successfully been imported and can be read vs documents that haven't because they errored, are invalid, etc.

The names might not be the best.

Some alternate possibilities:

• Saved / Unsaved
• Indexed / NonIndexed
• Imported / NonImported