Fix planet stealth tech cost

[o01eg]

Fixes #4127

[Grummel7]

Just had a closer look at it:
Pedia says, cost is reduced by cost of the ship part, which whoever is wrong for when you have a sneaky species. The sneaky reduction is missing.
Actually what the code does is it halfs the cost if:

• ship part has been researched
• empire owns a sneaky species.

I suggest to update the pedia accordingly and also implement it, using / (1 + Statistic If... notation for both.

[o01eg]

@Grummel7 I suppose it should be other PR.