[ Observatory ] Create fork of runtime/observatory at pkg/observatory

This change further prepares for the removal of Observatory from the
shipped Dart SDK by creating a fork to be used by the remaining
Observatory users.

This is basically a straight copy of the contents from
runtime/observatory with the exception of two new scripts:

- `bin/observatory.dart`, a utility to launch Observatory
- `bin/activate.dart`, which globally activates `bin/observatory.dart`
  as `observatory`

This change also updates the presubmits to ensure that
`runtime/observatory` is effectively placed in read-only mode to prevent
any divergences with the fork.

Work towards https://github.com/dart-lang/sdk/issues/50233

Change-Id: Iff3a7512058f36afa2a96d45d94a1dff424401d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429800
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

PRESUBMIT.py

@@ -537,6 +537,19 @@ def _CheckDartApiWinCSync(input_api, output_api):
     return []
 
 
+def _CheckNoRuntimeObservatoryChanges(input_api, output_api):
+    """Ensures that no further changes are made to runtime/observatory."""
+    for f in input_api.AffectedFiles(include_deletes=False):
+        path = f.LocalPath()
+        if path.startswith("runtime/observatory/"):
+            return [
+                output_api.PresubmitError(
+                    'Observatory is being moved to pkg/observatory. Files under '
+                    'runtime/observatory should no longer be modified.')
+            ]
+    return []
+
+
 def _CommonChecks(input_api, output_api):
     results = []
     results.extend(_CheckValidHostsInDEPS(input_api, output_api))
@@ -552,6 +565,7 @@ def _CommonChecks(input_api, output_api):
     results.extend(_CheckAnalyzerFiles(input_api, output_api))
     results.extend(_CheckDevCompilerSync(input_api, output_api))
     results.extend(_CheckDartApiWinCSync(input_api, output_api))
+    results.extend(_CheckNoRuntimeObservatoryChanges(input_api, output_api))
     return results