Check if receiver is a module object in relativePathTo (#1650)

Closes #1649

pkl-core/src/test/files/LanguageSnippetTests/input/errors/relativePathToNotAModule1.pkl

@@ -0,0 +1,7 @@
+local objectThatAmendsModule = (module) {
+  // add a local property to force this into a new object amends;
+  // empty object amends are optimized away.
+  local qux = 1
+}
+
+res = objectThatAmendsModule.relativePathTo(module)

pkl-core/src/test/files/LanguageSnippetTests/input/errors/relativePathToNotAModule2.pkl

@@ -0,0 +1,8 @@
+local objectThatAmendsModule = (module) {
+  // add a local property to force this into a new object amends;
+  // empty object amends are optimized away.
+  local qux = 1
+}
+
+res = module.relativePathTo(objectThatAmendsModule)
+

pkl-core/src/test/files/LanguageSnippetTests/output/errors/relativePathToNotAModule1.err

@@ -0,0 +1,14 @@
+–– Pkl Error ––
+Expected a module as the receiver, but got an object that amends a module.
+
+x | res = objectThatAmendsModule.relativePathTo(module)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at relativePathToNotAModule1#res (file:///$snippetsDir/input/errors/relativePathToNotAModule1.pkl)
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/relativePathToNotAModule2.err

@@ -0,0 +1,14 @@
+–– Pkl Error ––
+Expected a module as argument, but got an object that amends a module.
+
+x | res = module.relativePathTo(objectThatAmendsModule)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at relativePathToNotAModule2#res (file:///$snippetsDir/input/errors/relativePathToNotAModule2.pkl)
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)