Organize apiviewgo code into packages

Currently, all business logic is stuffed into the `cmd` package. The code would be easier to navigate, and boundaries between types would be easier to maintain, if concerns were separated into subpackages. Something like:
```
cmd
├── cache
├── content
├── module
└── pkg
```
This would be a nontrivial refactoring because types in `cmd` commonly manipulate each others' unexported state.

Start by reading the apiviewgo `cmd` package and mapping its business-logic types and their unexported-state dependencies. Use the proposed `cache`, `content`, `module`, and `pkg` subpackages as the target boundaries; done means the concerns are separated while their cross-package interactions remain functional.

Written by the indexing model from the issue text.