Fix some small spec typos and omissions

[lukewagner]

This PR fixes some small typos and ambiguities in spec and test suite. Specifically

• dangling async? immediate on resource.drop description is removed
• outer aliases can refer to outer imports (of the right sort) and what that means when unbundling
• import/export subtyping applies to core modules (in addition to components and instances)
• valtype changed to core:valtype in a few obvious places (that impls already get right)
• fix old copypasta in resource.* built-ins
• external visibility rules allow aliases of imports/exports, not just the direct typeidx of them