Cannot use optional: true with missing file wrapper resources

For resources that either need to get placed in PBXVariantGroups, or are otherwise represented as a directory, explicitly listing all the files in the directory and specifying optional: true does not give the resources proper handling.

For example, for a target with the following in its sources

{ "path": "../Frameworks/ST/Resources/Base.lproj/MQPSView.xib", "group": "Frameworks/ST/Resources/Base.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/PS.xcdatamodeld/PS.xcdatamodel", "group": "Frameworks/ST/Resources/PS.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/PS.xcdatamodeld/PS.xcdatamodel/contents", "group": "Frameworks/ST/Resources/PS.xcdatamodeld/PS.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/PY.xcdatamodeld/.xccurrentversion", "group": "Frameworks/ST/Resources/PY.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/PY.xcdatamodeld/PY.xcdatamodel", "group": "Frameworks/ST/Resources/PY.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/PY.xcdatamodeld/PY.xcdatamodel/contents", "group": "Frameworks/ST/Resources/PY.xcdatamodeld/PY.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/.xccurrentversion", "group": "Frameworks/ST/Resources/SP.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP 2.xcdatamodel", "group": "Frameworks/ST/Resources/SP.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP 2.xcdatamodel/contents", "group": "Frameworks/ST/Resources/SP.xcdatamodeld/SP 2.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP 3.xcdatamodel", "group": "Frameworks/ST/Resources/SP.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP 3.xcdatamodel/contents", "group": "Frameworks/ST/Resources/SP.xcdatamodeld/SP 3.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP.xcdatamodel", "group": "Frameworks/ST/Resources/SP.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/SP.xcdatamodeld/SP.xcdatamodel/contents", "group": "Frameworks/ST/Resources/SP.xcdatamodeld/SP.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/TL.xcdatamodeld/TL.xcdatamodel", "group": "Frameworks/ST/Resources/TL.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/TL.xcdatamodeld/TL.xcdatamodel/contents", "group": "Frameworks/ST/Resources/TL.xcdatamodeld/TL.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/T.xcdatamodeld/.xccurrentversion", "group": "Frameworks/ST/Resources/T.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/T.xcdatamodeld/T.xcdatamodel", "group": "Frameworks/ST/Resources/T.xcdatamodeld", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/T.xcdatamodeld/T.xcdatamodel/contents", "group": "Frameworks/ST/Resources/T.xcdatamodeld/T.xcdatamodel", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-AU.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/en-AU.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-AU.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/en-AU.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-CA.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/en-CA.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-CA.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/en-CA.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-GB.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/en-GB.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-GB.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/en-GB.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-IN.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/en-IN.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en-IN.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/en-IN.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/en.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/en.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/en.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/es-MX.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/es-MX.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/es-MX.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/es-MX.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/es.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/es.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/es.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/es.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/fr-CA.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/fr-CA.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/fr-CA.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/fr-CA.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/fr.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/fr.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/fr.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/fr.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/ja.lproj/Localizable.strings", "group": "Frameworks/ST/Resources/ja.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/ja.lproj/MQPSView.strings", "group": "Frameworks/ST/Resources/ja.lproj", "buildPhase": "none", "optional": true },
{ "path": "../Frameworks/ST/Resources/local.cert", "group": "Frameworks/ST/Resources", "buildPhase": "none", "optional": true },

the lprojs do not lead to a variant group, the datamodeld directories do not yield a XCVersionGroup, and (if there were any), asset catalogs would not be a bare file reference, but instead the contents.json would be added as a file reference.

This makes generating the project without access to the source files impossible

No source file or test entry point is named; begin by tracing XcodeGen's resource handling for the listed optional entries during project generation. Done means generation without source files still produces variant groups, XCVersionGroup entries, and correct asset-catalog references for directory-like resources.