Nowarn extension matching nonpublic member

[som-snytt]

Fixes #21816

Consider only public members when checking if an extension method actually extends.

A use site lint would be more thorough, when an extension is in scope but unused because of a visible matching member.

[som-snytt]

Possibly, a private[p] extension could warn if there is a private[p] member.

[smarter]

This could be member.symbol.isPublic but we can also be more precise and check for accessibility (untested):

Suggested change

	val memberIsPublic = (member.symbol.flags & AccessFlags).isEmpty && !member.symbol.privateWithin.exists
	memberIsPublic && {
	val memberIsAccessible = member.symbol.isAccessibleFrom(target)
	memberIsAccessible && {

[som-snytt]

I'll take another look during daylight, as my first swing was during interrupted sleep. I think the use case is an arbitrary use site, so that is the reason to require isPublic. I'll reconsider after coffee.