Resolve symbols from attached and imported packages

[lionel-]

Branched from #1257
Progress towards #1212

• File::resolve() and resolve_at() now handle the Package (NAMESPACE import()) and From (importFrom()) import layers (previously only File layers were walked; the rest were dropped).

• Package::resolve() answers "is this name exported by this package?" by walking the package's files and collecting their exports. It provides a lazy / post-load view of package exports. For instance it returns two definitions when a top-level stub is overwritten in the .onLoad() hook via <<-.

• Package::files are now stored in collation order, which simplifies downstream calls that need to respect R's load order.

File::imports() and Package::resolve() are both used in File::resolve() / File::resolve_at(), which themselves support goto-def, find-refs, and rename.

With these changes, when a workspace contains both scripts and a package, you can now use goto-def, find-refs, and rename with symbols imported via library().

goto-def will also work with installed packages when oak_sources is wired up again.