Non-Minecraft dependencies
Non-Minecraft dependencies are not loaded by NeoForge by default in the development environment. To get NeoForge to recognize the non-Minecraft dependency, they must be added to runs as a runtime dependency.
For example, you can add the com.example:example library to all runs like so:
dependencies {
    implementation 'com.example:example:1.0'
}
runs {
    configureEach {
        dependencies {
            runtime 'com.example:example:1.0'
        }
    }
}
Or, you can use a configuration:
configurations {
    libraries
    // This will make sure that all dependencies that you add to the libraries configuration will also be added to the implementation configuration
    // This way, you only need one dependency declaration for both runtime and compile dependencies
    implementation.extendsFrom libraries
}
dependencies {
    libraries 'com.example:example:1.0'
}
runs {
    configureEach {
        dependencies {
            runtime project.configurations.libraries
        }
    }
}
tip
configureEach will configure each run. If you instead only want to add a runtime dependency to one specific run you can use named(<name>).configure:
runs {
    // Only configure dependencies for the client run
    named('client').configure {
        dependencies {
        }
    }
}