Make Base.get_extension private to prevent user mistakes

Added in #58108 without valid justification: something
simply being documented in the manual is never any allowable argument
for making something public. Being public is confusing users, since all
of the other related infrastructure is also declared private (project
files, `loaded_modules_array`, `PkgId`, `require`, etc.). Some of those
do have legit uses in reflection testing, but we don't currently have a
representation for this sort of private-except-reflection in most places
(other than `Core.IR`).

However is not allowed for packages to call `Base.get_extension`, since
it requires accessing private implementation details of the target
package to construct the arguments, and can run afoul of various
implementation rules (world ages, module identity, precompile file
consistency) that can lead to buggy execution.

The correct way to use this functionality is with dispatch. Loosely:
```julia
module Main
function get_extension end
end

module MainPkgExt # extension module in Main
using Other
Main.get_extension(args...) = Other.call(args...)
end
```

Author: vtjnash

base/loading.jl

@@ -1643,7 +1643,9 @@ end
 """
     get_extension(parent::Module, extension::Symbol)
 
-Return the module for `extension` of `parent` or return `nothing` if the extension is not loaded.
+Return the module for `extension` relative to `parent` or return `nothing` if the extension is not loaded.
+This function is private, since the arguments to it are private implementation details.
+It is provided mainly for testing and reflection purposes.
 """
 get_extension(parent::Module, ext::Symbol) = get_extension(PkgId(parent), ext)
 function get_extension(parentid::PkgId, ext::Symbol)

base/public.jl

@@ -54,7 +54,6 @@ public
     active_project,
 
 # Reflection and introspection
-    get_extension,
     isambiguous,
     isexpr,
     isidentifier,