Make SIP 54 a standard feature

 - Drop experimental language import
 - Change note in docs to say that this relaxation only applies
   to extension method calls, not extension methods called as normal
   methods.

I tried to also reflect the second point in error messages but it turned out too hard.
At the point where we generate the error message we do not know how the method was called and
it would be unsystematic to create that side channel. In fact, information flows the other
way: When we resolve an extension method name, we buffer the error messages and fix selected
AmbiguityErrors.

Author: odersky

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -29,7 +29,6 @@ object Feature:
   val fewerBraces = experimental("fewerBraces")
   val saferExceptions = experimental("saferExceptions")
   val clauseInterleaving = experimental("clauseInterleaving")
-  val relaxedExtensionImports = experimental("relaxedExtensionImports")
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
   val into = experimental("into")

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -260,7 +260,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           then
             altImports.uncheckedNN += altImp
 
-        if Feature.enabled(Feature.relaxedExtensionImports) && altImports != null && ctx.isImportContext then
+        if altImports != null && ctx.isImportContext then
           val curImport = ctx.importInfo.uncheckedNN
           namedImportRef(curImport) match
             case altImp: TermRef =>

docs/_docs/reference/contextual/extension-methods.md

@@ -254,7 +254,8 @@ The following two rewritings are tried in order:
     not a wildcard import, pick the expansion from that import. Otherwise, report
     an ambiguous reference error.
 
-    **Note**: This relaxation is currently enabled only under the `experimental.relaxedExtensionImports` language import.
+    **Note**: This relaxation of the import rules applies only if the method `m` is used as an extension method. If it is used as a normal method in prefix form, the usual import rules apply, which means that importing `m` from
+    multiple places can lead to an ambiguity error.
 
  2. If the first rewriting does not typecheck with expected type `T`,
     and there is an extension method `m` in some eligible object `o`, the selection is rewritten to `o.m[Ts](e)`. An object `o` is _eligible_ if

library/src/scala/runtime/stdLibPatches/language.scala

@@ -69,14 +69,6 @@ object language:
     @compileTimeOnly("`clauseInterleaving` can only be used at compile time in import statements")
     object clauseInterleaving
 
-    /** Adds support for relaxed imports of extension methods.
-      * Extension methods with the same name can be imported from several places.
-      *
-      * @see [[http://dotty.epfl.ch/docs/reference/contextual/extension-methods]]
-      */
-    @compileTimeOnly("`relaxedExtensionImports` can only be used at compile time in import statements")
-    object relaxedExtensionImports
-
     /** Experimental support for pure function type syntax
      *
      *  @see [[https://dotty.epfl.ch/docs/reference/experimental/purefuns]]