Follow ups for file-based programs source package

Follow up to #51373

- [x] Usage of GracefulException, and associated compilation symbol, should be removed: https://github.com/dotnet/sdk/pull/51373#discussion_r2452723098
- [ ] Behavior should not depend on process current directory: https://github.com/dotnet/sdk/pull/51373#discussion_r2452734298
- [ ] Add a targets file to avoid need for the consumer to explicitly reference the resx item: https://github.com/dotnet/sdk/pull/51373#discussion_r2464921558
- [ ] Cleanup the target which excludes the resx generated output from the package: https://github.com/dotnet/sdk/pull/51373#discussion_r2464932605
- [ ] Delete copies of methods from `MsbuildProject` and `MSBuildUtilities` from the source package, sharing the original method(s) under `src/Cli` instead: https://github.com/dotnet/sdk/pull/54947
- [x] Consider reducing internal API usage by using `file class`es: https://github.com/dotnet/sdk/pull/51373#discussion_r2488753413
- [ ] Adjust `FileLevelDirectiveHelpers.FindLeadingDirectives` to avoid the need for caller to pass a `SourceText`: https://github.com/dotnet/roslyn/pull/80575#discussion_r2500697258
- [ ] Delete `ExternalHelpers.CombineHashCodes`: https://github.com/dotnet/roslyn/pull/80575#discussion_r2500724670