Return a useful object from `create_app` & co.

There is some useful stuff that is now only outputted via log messages. For example, when a precompile execution script is run, PackageCompiler prints:

```
[ Info: PackageCompiler: Executing /home/kc/JuliaPkgs/PackageCompiler.jl/precompile_example.jl => /tmp/jl_packagecompiler_gtWkhz/jl_cZBXfb
```

However, there is no good way to get at that log file with the precompile statements without basically copy-pasting it. Instead, `create_sysimage` could return an object where this stuff is stored. So a potential API for this specific case would be

```jl
info = create_sysimage(...)
for data in info.precompile_data
print(data.precompile_execution_file , " logged precompile statements to ", data.precompile_statements_log)
end
```

Start by reading the create_sysimage and create_app APIs, then trace where precompile execution and log paths are produced. Define the returned information object around precompile_execution_file and precompile_statements_log, and verify it with the precompile execution script example.

Written by the indexing model from the issue text.