Import Aliasing: Use if basename doesn't match

This adds a note to the import aliasing section that import aliasing
must be used if the base name doesn't match the package name.

This is placed before the more general recommendation of avoiding import
aliasing otherwise because it's more important to use named imports when
the basename doesn't match.

style.md

@@ -182,7 +182,7 @@ sPtrs[1].Write("test")
 ```
 
 Similarly, an interface can be satisfied by a pointer, even if the method has a
-value receiver. Example from the Go at Uber presentation:
+value receiver.
 
 ```go
 type F interface {
@@ -1168,8 +1168,20 @@ for the purpose of grouping related test cases, e.g.,
 
 ### Import Aliasing
 
-Import aliases should be avoided unless there is a direct conflict between
-imports.
+Import aliasing must be used if the package name does not match the last
+element of the import path.
+
+```go
+import (
+  "net/http"
+
+  client "example.com/client-go"
+  trace "example.com/trace/v2"
+)
+```
+
+In all other scenarios, import aliases should be avoided unless there is a
+direct conflict between imports.
 
 <table>
 <thead><tr><th>Bad</th><th>Good</th></tr></thead>
@@ -1201,9 +1213,6 @@ import (
 </td></tr>
 </tbody></table>
 
-<!-- TODO: named imports should always be used if basename does not match
-package name. -->
-
 ### Function Grouping and Ordering
 
 - Functions should be sorted in rough call order.