Patches that change packages or imports only

[abhinav]

gopatch requires exactly one entity after the package name (if any) and imports.
Patches that affect only imports are not supported.

This fails:

@@
@@
-import "example.com/foo-go"
+import "example.com/foo"

We can work around this by "using" foo in the patch without changing it.

@@
@@
-import "example.com/foo-go"
+import "example.com/foo"

 foo

We should not have this requirement, and instead support patches that don't affect code beyond the imports and packages.

This is likely related to #4 where we plan to add support for multiple entities in a single diff.