wasm: fix functions exported through //export

When a function is exported using //export, but also had a
//go:wasm-module pragma, the //export name was ignored. The
//go:wasm-module doesn't actually do anything besides breaking the
export (exported functions don't have a module name).

I've refactored and cleaned up the code, and in the process removed this
weird edge case.

compiler/testdata/pragma.go

@@ -62,6 +62,19 @@ func exportedFunctionInSection() {
 //go:wasmimport modulename import1
 func declaredImport()
 
+// Legacy way of importing a function.
+//
+//go:wasm-module foobar
+//export imported
+func foobarImport()
+
+// The wasm-module pragma is not functional here, but it should be safe.
+//
+//go:wasm-module foobar
+//export exported
+func foobarExportModule() {
+}
+
 // This function should not: it's only a declaration and not a definition.
 //
 //go:section .special_function_section