Support page_width from analysis_options in builders

[schultek]

Not sure if this is better opened here or in the build repository. Please move where you prefer it.

I'd like to update my build_runner packages to use the new project-wide page_width setting for generated Dart code. Currently packages either generate unformatted code, or need to hardcode a page-width value or use a proprietary configuration.
Using the new project wide formatter configuration would enable all build_runner packages to always generate correctly formatted code based on the users preference.

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For example, I'm using a custom build options property 'lineLength':

var pageWidth = options.lineLength;

var formatter = DartFormatter(
  languageVersion: Version(3, 7, 0),
  pageWidth: pageWidth,
);

await buildStep.writeAsString(outputId, formatter.format(source));

What I'd like to do is this:

var pageWidth = options.lineLength;

if (pageWidth == null) {
  // This doesn't exist.
 pageWidth = DartFormatter.findPageWidthFromAnalysisOptions();
}

var formatter = DartFormatter(
  languageVersion: Version(3, 7, 0),
  pageWidth: pageWidth,
);

await buildStep.writeAsString(outputId, formatter.format(source));

However I didn't find any way to do it.

Looking through this packages source, I found the ConfigCache().findPageWidth(...) method, but this accepts a File and build strongly discourages any file based io, mainly by not giving me any file paths I could use here (it only has AssetIds).