Package-dependent compiler flags

Adding `{"dependency": "core"}` to `findlibPackages` will cause failure in a build, because the package demands either the `-threads` or `-vmthread` flag. I added a bit of code to be able to group these kinds of flags with their dependencies:

```
, "findlibPackages": [{"dependency": "lambdasoup"}
, {"dependency": "core"
, "compileFlags": [" -vmthread "]}
, {"dependency": "cohttp"}
, {"dependency": "cohttp.lwt"}
, {"dependency": "lwt"}]
```

I'm not 100% sure it's a great idea. I like the explicit nature of this, and if different packages have conflicting packages, it's easier to point out as an exception. But compiler flags feel like a higher-level concept than packages, so I'm not sure about it.

Start by tracing how findlibPackages entries are interpreted, focusing on the proposed compileFlags field and the package-dependent flags shown in the issue. Determine whether this representation should be adopted and define completion as a documented, working approach for builds requiring -threads or -vmthread, including handling conflicting package flags.

Written by the indexing model from the issue text.