Follow ups for file-based programs source package

[RikkiGibson]

Follow up to #51373

• Usage of GracefulException, and associated compilation symbol, should be removed: Add source package for file-based programs support #51373 (comment)
• Behavior should not depend on process current directory: Add source package for file-based programs support #51373 (comment)
• Add a targets file to avoid need for the consumer to explicitly reference the resx item: Add source package for file-based programs support #51373 (comment)
• Cleanup the target which excludes the resx generated output from the package: Add source package for file-based programs support #51373 (comment)
• Delete copies of methods from MsbuildProject and MSBuildUtilities from the source package, sharing the original method(s) under src/Cli instead.
• Consider reducing internal API usage by using file classes: Add source package for file-based programs support #51373 (comment)
• Adjust FileLevelDirectiveHelpers.FindLeadingDirectives to avoid the need for caller to pass a SourceText: File-based programs live directive diagnostics roslyn#80575 (comment)
• Delete ExternalHelpers.CombineHashCodes: File-based programs live directive diagnostics roslyn#80575 (comment)