Avoid depending on files from build_system/targets other than from top level entrypoints in flutter_tools. (#142760)

Add a new `BuildTargets` class that provides commonly used build targets. And avoid importing files from `build_system/targets` except from the top level entrypoints or from top level commands.

Also move `scene_importer.dart` and `shader_compiler.dart` into `build_system/tools` because they are not `Target` classes, but wrapper for certain tools.

With this change, we can ignore all files in `build_system/targets` internally and make PR #142709 easier to land internally. See cl/603434066 for the corresponding internal change.

Related to:
https://github.com/flutter/flutter/pull/142709
https://github.com/flutter/flutter/issues/142041

Also note that I have opted to add a new variable in `globals.dart` for `BuildTargets` in this PR, but I know that we are trying to get rid of globals. Several alternatives that I was considering:

1. Add a new field in `BuildSystem` that returns a `BuildTargets` instance. Since `BuildSystem` is already in `globals`, we can access build targets using `globals.buildSystem.buildTargets` without adding a new global variable.
2. Properly inject the `BuildTargetsImpl` instance from the top level `executable.dart` and top level commands.

Let me know if you want me to do one of the above instead. Thanks!