cortex.css file existence checked before running pre-build script

[rhgb]

The problem issued in #478 closed by 0e3b9fc reappeared after committing 89ec5ad:

       'read_cortex_json',
 +      'clean_cortex_json',    // this line is originally located after 'run_prebuild_script'
 +      'prepare_build_type',
        'run_preinstall_script',
        'run_prebuild_script',

so what's the intention of 89ec5ad and how should we fix it again?

[chinesedfan]

clean_cortex_json will set pkg.css to be an empty array if no files were found. See cortexjs/read-cortex-json#8.

I think the right order is run_prebuild_script -> clean_cortex_json -> prepare_build_type.

[chinesedfan]

But run_prebuild_script needs prepare_build_type. Because if build_type != 'full', we can skip run_prebuild_script. Then these 3 steps become a loop. :(