Consider moving off of package:stager

After pulling from the latest master of devtools, I received the following error:

```
Because devtools_app depends on integration_test from sdk which depends on test_api 0.7.4, test_api 0.7.4 is required.
And because test 1.25.8 depends on test_api 0.7.3, test 1.25.8 is forbidden.
And because devtools_tool depends on test ^1.25.8 and no versions of test match >1.25.8 <1.25.9, test ^1.25.9 is required.
And because test >=1.25.9 depends on analyzer >=6.0.0 <8.0.0 and stager 1.0.1 depends on analyzer ^5.1.0, stager 1.0.1 is forbidden.
Because no versions of stager match >1.0.1 <2.0.0 and devtools_app depends on stager ^1.0.1, stager 1.0.1 is required.
Thus, version solving failed.
```

The workaround is to add the following dependency_overrides to the top level pubspec.yaml:
```
dependency_overrides:
analyzer: 6.11.0
dart_style: 2.3.3
```

The longer term fix will require stager to update it's dependencies:
https://github.com/bryanoltman/stager/pull/48

We should make a decision if we should continue to depend on stager going forward if the versioning issue cannot be addressed from the stager side.

Start with the top-level pubspec.yaml and reproduce the reported dependency resolution conflict involving stager, analyzer, test_api, and dart_style. Review stager PR #48 and determine whether the dependency can be updated or stager should be removed; done means recording a clear decision and resolving the versioning issue without the reported workaround.

Scritto dal modello di indicizzazione a partire dal testo della issue.