By using the input map as the basis, which means the hash will not change if we add/rename/remove options.
This happened in Hugo 0.99, as we added a new options. This is unortunate.
Unfortunately this means that the cache keys for PostCSS will change one more time in 0.100, but will be stable going forward.
Note that we have implemented this pattern in all the other resource transformers.
Updates #9787
Note that we will now fail if `inlineImports` is enabled and we cannot resolve an import.
You can work around this by either:
* Use url imports or imports with media queries.
* Set `skipInlineImportsNotFound=true` in the options
Also get the argument order in the different NewFileError* funcs in line.
Fixes#9895
