WorkspaceSymbols index after successful build

[axelson]

Previously WorkspaceSymbols depended on a side-effect of running Dialyzer:
#110 (comment)

Now that the Build step loads all modules (added in #227) we need to change the WorkspaceSymbols index to be rebuilt after a successful compile.

[axelson]

Need to fix the test error:

** (Mix.Error) Cannot access build without an application name, please ensure you are in a directory with a mix.exs file and it defines an :app name under the project configuration

    (mix) lib/mix.ex:323: Mix.raise/1

    (mix) lib/mix/project.ex:521: Mix.Project.compile_path/1

    (mix) lib/mix/tasks/compile.all.ex:24: anonymous fn/1 in Mix.Tasks.Compile.All.run/1