test(module): add import resolution tests

src/test/kotlin/lumen/ImportTest.kt

@@ -0,0 +1,65 @@
+package lumen
+
+import java.io.File
+import kotlin.io.path.createTempDirectory
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class ImportTest {
+    private fun runModules(files: Map<String, String>, main: String): String {
+        val dir = createTempDirectory("lumen-import").toFile()
+        try {
+            for ((name, source) in files) {
+                val file = File(dir, name)
+                file.parentFile.mkdirs()
+                file.writeText(source)
+            }
+            val buffer = StringBuilder()
+            Lumen.runFile(File(dir, main), buffer)
+            return buffer.toString()
+        } finally {
+            dir.deleteRecursively()
+        }
+    }
+
+    @Test
+    fun importBringsDefinitionsIntoScope() {
+        val output = runModules(
+            mapOf(
+                "lib/util.lumen" to "fn double(x) { return x * 2 }\nlet unit = 1",
+                "main.lumen" to "import \"lib/util.lumen\"\nprintln(double(21))\nprintln(unit)"
+            ),
+            "main.lumen"
+        )
+        assertEquals("42\n1\n", output)
+    }
+
+    @Test
+    fun diamondImportLoadsSharedModuleOnce() {
+        val output = runModules(
+            mapOf(
+                "base.lumen" to "println(\"loading base\")\nfn origin() { return \"base\" }",
+                "left.lumen" to "import \"base.lumen\"\nfn left() { return origin() }",
+                "right.lumen" to "import \"base.lumen\"\nfn right() { return origin() }",
+                "main.lumen" to "import \"left.lumen\"\nimport \"right.lumen\"\nprintln(left() + right())"
+            ),
+            "main.lumen"
+        )
+        val baseLoads = output.split("\n").count { it == "loading base" }
+        assertEquals(1, baseLoads, "shared module must run exactly once")
+        assertEquals("loading base\nbasebase\n", output)
+    }
+
+    @Test
+    fun mutualImportsDoNotLoopForever() {
+        val output = runModules(
+            mapOf(
+                "a.lumen" to "import \"b.lumen\"\nfn fromA() { return \"A\" }",
+                "b.lumen" to "import \"a.lumen\"\nfn fromB() { return \"B\" }",
+                "main.lumen" to "import \"a.lumen\"\nprintln(fromA() + fromB())"
+            ),
+            "main.lumen"
+        )
+        assertEquals("AB\n", output)
+    }
+}