Be sure to GC our imports as well as the module

After a module goes through its primary GC pass we need to look over the
set of remaining imports and use that to prune the set of imports that
we're binding.

Closes #1613

crates/cli-support/src/js/mod.rs

@@ -204,8 +204,18 @@ impl<'a> Context<'a> {
         // After all we've done, especially
         // `unexport_unused_internal_exports()`, we probably have a bunch of
         // garbage in the module that's no longer necessary, so delete
-        // everything that we don't actually need.
+        // everything that we don't actually need. Afterwards make sure we don't
+        // try to emit bindings for now-nonexistent imports by pruning our
+        // `wasm_import_definitions` set.
         walrus::passes::gc::run(self.module);
+        let remaining_imports = self
+            .module
+            .imports
+            .iter()
+            .map(|i| i.id())
+            .collect::<HashSet<_>>();
+        self.wasm_import_definitions
+            .retain(|id, _| remaining_imports.contains(id));
 
         // Cause any future calls to `should_write_global` to panic, making sure
         // we don't ask for items which we can no longer emit.

crates/cli/tests/wasm-bindgen/main.rs

@@ -136,3 +136,18 @@ fn works_on_empty_project() {
 }
 
 mod npm;
+
+#[test]
+fn one_export_works() {
+    let (mut cmd, _out_dir) = Project::new("one_export_works")
+        .file(
+            "src/lib.rs",
+            r#"
+                use wasm_bindgen::prelude::*;
+                #[wasm_bindgen]
+                pub fn foo() {}
+            "#,
+        )
+        .wasm_bindgen("");
+    cmd.assert().success();
+}