chore: support custom autoModules

support custom autoModules like css-loader-auto

the CircleCi use node v16 which cause the CI failed

Just stumbled upon this because my project only knows CSS modules, thus the file tag can be omitted (which rollup-plugin-postcss per default does not allow). However, it can be achieved by disabling autoModules and setting modules to true:

{
    // Disable `autoModules` and always enable `modules` (so that all input files are treated as modules)
    autoModules: false,
    modules: true
})

Just leaving this bit of information here in case anybody is looking for the same functionality.