[flutter_svg] Initial import

Imports https://github.com/dnfield/flutter_svg into this repository,
with history, and updates it to follow repository conventions:
- Updates min SDKs.
- Removes analysis options and fixes resulting warnings.
- Autoformats.
- Updates pubspecs to follow repo standard.
- Updates repo tooling to allow the `flutter_svg_test` non-dev
  dependency on `flutter_test`.
- Adds repo metadata.
- Adds METADATA files.
- Adds commemoration to README.
- Updates example app Android build files to current standards.
- Bumps versions and slightly relax version constraints for
  vector_graphics* to allow the new versions.
- Moves gitignore of golden test diffs into the package.

script/tool/lib/src/pubspec_check_command.dart

@@ -569,8 +569,10 @@ class PubspecCheckCommand extends PackageLoopingCommand {
       'test',
     };
     // Non-published packages like pigeon subpackages are allowed to violate
-    // the dev only dependencies rule.
-    if (pubspec.publishTo != 'none') {
+    // the dev only dependencies rule, as are packages that end in `_test` (as
+    // they are assumed to be intended to be used as dev_dependencies by
+    // clients).
+    if (pubspec.publishTo != 'none' && !pubspec.name.endsWith('_test')) {
       pubspec.dependencies.forEach((String name, Dependency dependency) {
         if (devOnlyDependencies.contains(name)) {
           misplacedDevDependencies.add(name);