libexpr: Move derivation-internal.nix from corepkgsFS to internalFS

Best I can tell this was never supposed to be exposed to the user
and has been this way since 2.19.

2.18 did not expose this file to the user:

nix run nix/2.18-maintenance -- eval --expr "import <nix/derivation-internal.nix>"

error: getting status of '/__corepkgs__/derivation-internal.nix': No such file or directory

Author: xokdvium

src/libexpr/eval.cc

@@ -268,7 +268,7 @@ EvalState::EvalState(
     }())
     , corepkgsFS(make_ref<MemorySourceAccessor>())
     , internalFS(make_ref<MemorySourceAccessor>())
-    , derivationInternal{corepkgsFS->addFile(
+    , derivationInternal{internalFS->addFile(
           CanonPath("derivation-internal.nix"),
 #include "primops/derivation.nix.gen.hh"
           )}

tests/functional/lang/eval-fail-derivation-name.err.exp

@@ -1,20 +1,20 @@
 error:
        … while evaluating the attribute 'outPath'
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at «nix-internal»/derivation-internal.nix:<number>:<number>:
      <number>|     value = commonAttrs // {
      <number>|       outPath = builtins.getAttr outputName strict;
              |       ^
      <number>|       drvPath = strict.drvPath;
 
        … while calling the 'getAttr' builtin
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at «nix-internal»/derivation-internal.nix:<number>:<number>:
      <number>|     value = commonAttrs // {
      <number>|       outPath = builtins.getAttr outputName strict;
              |                 ^
      <number>|       drvPath = strict.drvPath;
 
        … while calling the 'derivationStrict' builtin
-         at <nix/derivation-internal.nix>:<number>:<number>:
+         at «nix-internal»/derivation-internal.nix:<number>:<number>:
      <number>|
      <number>|   strict = derivationStrict drvAttrs;
              |            ^