zig fetch: add --save flag

```
--save        Add the fetched package to build.zig.zon
--save=[name] Add the fetched package to build.zig.zon as name
```

doc/build.zig.zon.md

@@ -61,7 +61,7 @@ String.
 
 When this is provided, the package is found in a directory relative to the
 build root. In this case the package's hash is irrelevant and therefore not
-computed.
+computed. This field and `url` are mutually exclusive.
 
 ### `paths`