Simplify integration test layout

[lihaoyi]

• test/src/ just becomes src/, cutting out one unnecessary degree of nesting
• repo/ is now resources/, letting us use the now-standard MILL_TEST_RESOURCE_FOLDER environment variable rather than the bespoke MILL_INTEGRATION_REPO_ROOT
• example.foo[bar] no longer extends JavaModule, and instead only defines the minimal set of properties necessary to configure the child local/fork/server modules correctly, and unlike the integration.* modules they do not need to compile anything themselves.
• The child ModeModules in both example and integration also no longer inherit MillJavaModule, since they do not compile anything themselves. Instead they inherit mill.scalalib.TestModule with the minimal set of properties wired up

This should greatly simplify things overall, as the various integration and example test modules no longer have a bunch of unwanted JavaModule/ScalaModule tasks that need to be overriden.