Target of `set_preferences` unclear

[vchuravy]

In JuliaParallel/MPI.jl#570 we noticed that the target of set_preferences is not the current active project,
but the project that contains the package in it's Project.toml.

The text in

Preferences.jl/src/Preferences.jl

Lines 188 to 191 in 80b5509

	The above snippet first clears the `"compiler_options"` key of any inheriting influence,
	then sets a preference option, which guarantees that future loading of that preference
	will be exactly what was saved here. If we wanted to re-enable inheritance from higher
	up in the chain, we could do the same but passing `missing` first.

is a bit confusing since it seemingly implies that => nothing will set at a lower level (e.g. the current Project) a clear project, but instead it is also targeting the Project higher up in the LOAD_PATH.

Should we just always target the current Project?

cc: @staticfloat

[staticfloat]

Fixed by #37