SE-0386: `package` access modifier

[taylorswift]

@tshortli

To illustrate why I think package and @_spi serve different purposes I'll again make an analogy with Obj-C/C framework headers:

Swift modifier	Obj-C/C header visibility	Intended audience
public	Public	all modules
@_spi(...) public	Private	authorized external modules
package	Project	modules in the same project

i don’t think that “modules in the same project” is a useful intended audience, because i often have example targets (or, for packages that require 5.7+, snippet targets) that i don’t want to be included in that audience.

i wouldn’t want code in an example target or an integration test target that compiles in the CI pipeline, but fails to compile when used in an actual project.