docs: update import grammar

Author: nathanielc

docs/SPEC.md

@@ -1460,7 +1460,7 @@ Module paths need not be unique across different registries.
 
 #### Registry attribute
 
-The `registry` attribute defines the available registries.
+The `registry` attribute defines the available registries and must preceed a package clause.
 The registry attribute expects two arguments, the first is the name of the registry and the second is the `$base` URL of the registry API endpoint.
 See the [Registry API](#registry-api) for more details.
 
@@ -1482,7 +1482,9 @@ The import path may contain specifiers about which registry and which versions o
 An import path follows this grammar:
 
     ImportPath       = ModulePath [ "/" PackagePath ] [ Version ] .
-    ModulePath       = PathElement [ "/" PathElement ] [ MajorVersion ] .
+    ModulePath       = RegistryName [ "/" ModuleName ] [ MajorVersion ] .
+    RegistryName     = PathElement
+    ModuleName       = PathElement
     MajorVersion     = "/v" int_lit .
     PackagePath      = PathElement { "/" PathElement } .
     Version          = PreVersion | MinVersion