Rename PosixSourceAccessor to OsSourceAccessor

[Ericson2314]

The name is confusing because it works on Windows too, as is the intention.

[roberth]

I would expect a subtype relation here.

PosixSourceAccessor could be the implementation of a NativeSourceAccessor interface, and almost all code would refer to NativeSourceAccessor, except perhaps code like the Linux-specific sandbox implementation, which might benefit from interacting with the more specific type.

[Ericson2314]

That could be a thing, but so far this is no need for the PosixSourceAccessor subtype! The SourceAccessor interface is the restricted file system object model, and that is valid on both platforms.

[Ericson2314]

Per #11178 (comment) thread and @roberth's and @L-as's suggestions, let's do Os not Native for these things.