Co-authored-by: Robin Gloster <mail@glob.in>

stdenv: print message if structuredAttrs is enabled

stdenv: add _append

reduces the chance of a user doing it wrong

fix nix develop issue

output hooks don't work yet in nix develop though

making $outputs be the same on non-structuredAttrs and structuredAttrs
is too much trouble.

lets instead make a function that gets the output names

reading environment file '/nix/store/2x7m69a2sm2kh0r6v0q5s9z1dh41m4xf-xz-5.2.5-env-bin'
nix: src/nix/develop.cc:299: std::string Common::makeRcScript(nix::ref<nix::Store>, const BuildEnvironment&, const Path&): Assertion `outputs != buildEnvironment.vars.end()' failed.

use a function to get all output names instead of using $outputs

copy env functionality from https://github.com/NixOS/nixpkgs/pull/76732/commits

derivations can be coerced to their output paths

so we don't have to add these to the env attrset

if theres a source $stdenv then this is needed

for structuredAttrs

attributes

nix-repl> legacyPackages.x86_64-linux.fastStdenv
error: The ‘env’ attribute set cannot contain any attributes passed to derivation. The following attributes are overlapping: cc
«derivation

so no need to use `or`

stdenv: error if using {prepend,append}ToVar on associative array

i don't know how to prepend to associative array