Cannot override template referenced from dictionary value

## Steps to reproduce

### Base.stg

```
Map ::= [
"a": {}
]

example() ::= "override this"
```

### Derived.stg

```
import "Base.stg"

main() ::= <<

>>

example() ::= "custom"
```

### Code

```java
STGroup group = new STGroupFile("Derived.stg");
ST template = group.getInstanceOf("a");
template.render();
```

## Expected result

```
custom
```

## Actual result

```
override this
```

Start with the Base.stg and Derived.stg reproducer, then trace the Java entry points STGroupFile, getInstanceOf, and render. Confirm that rendering the dictionary value currently produces "override this" and that done means it produces the derived template's "custom" output.

Written by the indexing model from the issue text.