Include module name for definitions files (#861)

Closes #818 

We set the `moduleName` of the source module to the provided
`packageName`.
This then populates the relevant `definitionModuleName`'s on
CTV/FTV/TTVs, so it allows us to lookup where they originated from.

The one place I can see this having an impact inside of Luau code is the
following:

https://github.com/Roblox/luau/blob/1fa8311a188a932233d2ce2bc6412e5bab37c035/Analysis/src/Error.cpp#L70-L90
Which will potentially change they way error messages are formatted.
Should there be a way to exclude definition files when calling
`getDefinitionModuleName`, so we can preserve this behaviour?

Analysis/src/Frontend.cpp

@@ -38,6 +38,7 @@ LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauReadWriteProperties, false)
 LUAU_FASTFLAGVARIABLE(LuauTypecheckLimitControls, false)
 LUAU_FASTFLAGVARIABLE(CorrectEarlyReturnInMarkDirty, false)
+LUAU_FASTFLAGVARIABLE(LuauDefinitionFileSetModuleName, false)
 
 namespace Luau
 {
@@ -165,6 +166,11 @@ LoadDefinitionFileResult Frontend::loadDefinitionFile(GlobalTypes& globals, Scop
     LUAU_TIMETRACE_SCOPE("loadDefinitionFile", "Frontend");
 
     Luau::SourceModule sourceModule;
+    if (FFlag::LuauDefinitionFileSetModuleName)
+    {
+        sourceModule.name = packageName;
+        sourceModule.humanReadableName = packageName;
+    }
     Luau::ParseResult parseResult = parseSourceForModule(source, sourceModule, captureComments);
     if (parseResult.errors.size() > 0)
         return LoadDefinitionFileResult{false, parseResult, sourceModule, nullptr};

tests/TypeInfer.definitions.test.cpp

@@ -441,4 +441,27 @@ TEST_CASE_FIXTURE(Fixture, "class_definitions_reference_other_classes")
     REQUIRE(result.success);
 }
 
+TEST_CASE_FIXTURE(Fixture, "definition_file_has_source_module_name_set")
+{
+    ScopedFastFlag sff{"LuauDefinitionFileSetModuleName", true};
+
+    LoadDefinitionFileResult result = loadDefinition(R"(
+        declare class Foo
+        end
+    )");
+
+    REQUIRE(result.success);
+
+    CHECK_EQ(result.sourceModule.name, "@test");
+    CHECK_EQ(result.sourceModule.humanReadableName, "@test");
+
+    std::optional<TypeFun> fooTy = frontend.globals.globalScope->lookupType("Foo");
+    REQUIRE(fooTy);
+
+    const ClassType* ctv = get<ClassType>(fooTy->type);
+
+    REQUIRE(ctv);
+    CHECK_EQ(ctv->definitionModuleName, "@test");
+}
+
 TEST_SUITE_END();