Make sure that we can’t build the same derivation twice at the same
time.

Fix https://github.com/NixOS/nix/issues/5029

nix repl: Update :edit doc to remove inaccurate use of "derivation"

It supports functions as well. Also change `package` to
`derivation` because it operates at the language level and does
not open the derivation (which would be useful but not nearly
as much).

It does not operate on a derivation and does not return a
derivation path. Instead it works at the language level,
where a distinct term "package" is more appropriate to
distinguish the parent object of `meta.position`; an
attribute which doesn't even make it into the derivation.

Use a dedicated make target for the man page rather than bundling the
generation as part of `install`.
Also make sure that `make install` is a fixpoint by

- Removing the generated markdown files from `MANUAL_SRCS`
- Not having the manpage generation write in its source directory so as
  to not update its timestamp (it would run each time otherwise)

Forward the whole Nix config to the repl subprocesses

Be more lenient when realisations have a conflicting dependency set

- This can legitimately happen (for example because of a non-determinism
  causing a build-time dependency to be kept or not as a runtime
  reference)
- Because of older Nix versions, it can happen that we encounter a
  realisation with an (erroneously) empty set of dependencies, in which
  case we don’t want to fail, but just warn the user and try to fix it.

Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the nix subprocesses in the repl
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available.

This is required for example to make `nix repl` work with a custom
`--store`

Forward the whole Nix config to the post-build-hook

Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the post-build-hook.
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available to the hook

nlohmann_json: Update to 3.9.1, fix use of internal copy

libexpr: Fix read out-of-bound on the heap

Signed-off-by: Pamplemousse <xav.maso@gmail.com>

Add a `devShells` attribute to the flake schema

Make `nix develop .#foo` search `.#devShells.${system}.foo` first

fixed-output derivations: fix incorrect responses for getpwuid

Passing nscd socket into the build environment causes unexpected behavior in programs that make getpwuid and other related calls.

relevant threads:
- https://github.com/NixOS/nix/issues/4991
- https://discourse.nixos.org/t/haunted-nix-build-breaks-isolation/13869

toString also coerces a set with an outPath attribute to a string

 Conflicts:
        src/nix/develop.cc
        src/nix/get-env.sh
        tests/shell.nix

'--delete-older-than 10' deletes the generations older than a single day, and '--delete-older-than 12m' deletes all generations older than 12 days.

This changes makes it throw on those invalid inputs, and gives an example of a valid input.

nix-repl> builtins.toString { outPath = "somestring"; }
"somestring"

nix print-dev-env: Add --json flag

nix develop: Make bash environment parsing more robust

Instead have get-env.sh dump the bash environment as JSON. This should
be a lot less error-prone.

Fixes #4992.