`UnnecessarilyFullyQualified` is inconsistent between `module-info.java` and `package-info.java`

`UnnecessarilyFullyQualified` occurs for `module-info.java`:

```java
@org.jspecify.annotations.NullMarked
module foo {}
```

but not for `package-info.java`:

```java
@org.jspecify.annotations.NullMarked
package bar;
```

This inconsistency is troublesome. I'd much prefer to have the check apply to all files, changing the behavior for `package-info.java` files. However, `package-info.java` files were explicitly excluded from the check in https://github.com/google/error-prone/issues/1652. Ideally there would be a flag to indicate whether to exclude `package-info.java` and `module-info.java` files from the check.

Locate the implementation and tests for the UnnecessarilyFullyQualified check, then review the package-info.java exclusion discussed in issue 1652. Decide how the exclusion flag should cover package-info.java and module-info.java, and add tests showing both file types behave consistently under each setting.

Written by the indexing model from the issue text.