lib/repo: Split archive/bare file parsing

lib/repo: Split archive/bare file parsing

Prep for future cleanup patches (in particular I want an internal-only
version at first that uses a fd+`struct stat`) to avoid allocations.

The new version avoids lots of deep nesting of conditionals as well
by hoisting the "not found" handling to an early return.

There's a bit of code duplication between the two cases but it's
quite worth the result.

Closes: #951
Approved by: jlebon