fix: default to root when module-dirs is not set #4163

Job	Run time
Console	51s
Shard Integration Tests	9s
Rebuild All	53s
VSCode Extension	41s
Lint	52s
Proto Breaking Change Check	8s
SQL	52s
Test Go	51s
Test README	53s
Integration Test (TestServiceWithRealDal)	22s
Integration Test (TestCron)	22s
Integration Test (TestFSM)	22s
Integration Test (TestFSMRetry)	36s
Integration Test (TestFSMGoTests)	22s
Integration Test (TestHttpIngress)	34s
Integration Test (TestLease)	21s
Integration Test (TestPubSub)	21s
Integration Test (TestDatabase)	23s
Integration Test (TestASMWorkflow)	23s
Integration Test (TestASMPagination)	22s
Integration Test (TestCmdsCreateProjectTomlFilesIfNonexistent)	23s
Integration Test (TestDefaultToRootWhenModuleDirsMissing)	21s
Integration Test (TestNonExportedDecls)	22s
Integration Test (TestUndefinedExportedDecls)	18s
Integration Test (TestHttpEncodeOmitempty)	22s
Integration Test (TestLifecycle)	20s
Integration Test (TestInterModuleCall)	22s
Integration Test (TestSchemaGenerate)	19s
Integration Test (TestModuleUnitTests)	20s
Integration Test (TestFTLMap)	22s
Integration Test (TestRuntimeReflection)	19s
Integration Success	1s
	14m 27s

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