chore: support custom autoModules

[2heal1]

support custom autoModules like css-loader-auto

[2heal1]

the CircleCi use node v16 which cause the CI failed

[filecage]

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.