Relax the uniqueness requirements on kebab-names (#259)

Resolves #252

Author: lukewagner

design/mvp/Binary.md

@@ -352,12 +352,8 @@ Notes:
   (which disallows core sorts other than `core module`). When the optional
   `externdesc` immediate is present, validation requires it to be a supertype
   of the inferred `externdesc` of the `sortidx`.
-* The `name` fields of `exportname` and `importname` must be unique among all
-  imports and exports in the containing component definition, component type or
-  instance type. (An import and export cannot use the same `name`.)
-* The `iid`, `pkgid` and `pkgidset` of `importname` and `exportname` must be
-  unique only among imports or exports. That is, two imports may *not*
-  have the same id, but an import and export *may* have the same id.
+* The `<name>`, `<iid>`, `<pkgid>` and `<pkgidset>` of imports must be relatively unique.
+* The `<name>` and `<iid>` of exports must be relatively unique.
 * `<iid>`, `<pkgid>` and `<pkgidset>` refer to the grammatical productions defined in
   the [text format](#import-and-export-definitions).
 * `<valid semver>` is as defined by [https://semver.org](https://semver.org/)

design/mvp/Explainer.md

@@ -1443,12 +1443,7 @@ options for naming imports:
   when the given exported definition.
 
 Export names include the same uniqueness requirements between exports as described
-above for import names and *additionally* require kebab-`name`s to be unique
-between imports and exports. This allows a single kebab name to uniquely select
-an import *or* export without an "import" or "export" prefix. (The same
-interface id can however be present as both an import and export, as is
-necessary for basic [virtualization](examples/LinkTimeVirtualization.md) use
-cases.)
+above for import names.
 
 As an example, the following component uses all 9 cases of imports and exports:
 ```wasm