javac should implement the following (optional) JLS 7.6 constraint as a warning:

  When packages are stored in a file system (?7.2.1), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:

    * The type is referred to by code in other compilation units of the package in which the type is declared.
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