Why does vscode think the declared package doesn't match the expected package?

Situation

I did the following with a project from GitHub

1. cloned the project: git clone https://github.com/antlr/antlr4.git
2. opened the project with: code antlr4
3. opened a file in the project: Antlr4MojoTest.java

The file is found under the following path in the project: antlr4/antlr4-maven-plugin/src/test/java/org/antlr/mojo/antlr4/Antlr4MojoTest.java

VSCode tells me: The declared package "org.antlr.mojo.antlr4" does not match the expected package "java.org.antlr.mojo.antlr4"

Question

Why does vscode think the expected package is java.org.antlr.mojo.antlr4?