Implement ctx.runfiles() entirely in Starlark

Right now this function calls into a native implementation that lives in
ConfiguredTarget. This one returns runfiles objects that are implemented
in Go.

This change gets rid of that, replacing runfiles objects with a simple
struct. The merge() and merge_all() methods are provided in the form of
computed fields.

Author: EdSchouten

pkg/model/analysis/BUILD.bazel

@@ -23,6 +23,7 @@ go_library(
         "configured_target.go",
         "directory_creation_parameters.go",
         "directory_readers.go",
+        "empty_default_info.go",
         "exec_transition.go",
         "file_access_parameters.go",
         "file_creation_parameters.go",

pkg/model/analysis/computer.json

@@ -127,6 +127,7 @@
             "ConfiguredTarget",
             "DirectoryCreationParametersObject",
             "FileCreationParametersObject",
+            "EmptyDefaultInfo",
             "ExecTransition",
             "ResolvedToolchains",
             "RootModule",
@@ -167,6 +168,16 @@
             "type": "*DirectoryReaders[TReference]"
          }
       },
+      "EmptyDefaultInfo": {
+         "dependsOn": [
+            "BuiltinsModuleNames",
+            "CanonicalRepoName",
+            "CompiledBzlFileDecodedGlobals",
+            "CompiledBzlFileFunctionFactory",
+            "CompiledBzlFileGlobal",
+            "RootModule"
+         ]
+      },
       "ExecTransition": {
          "dependsOn": [
             "BuiltinsModuleNames",