Fix #4823

Resolve the derivation before trying to load its environment −
essentially reproducing what the build loop does − so that we can
effectively access our dependencies (and not just their placeholders).

Fix #4821

This prevents spaces or other metacharacters from causing nix-shell to
execute the wrong path.

Fixes #4229.

Co-authored-by: Théophane Hufschmitt <regnat@users.noreply.github.com>

All GC roots are now indirect.

This is an alternative to the IN_NIX_SHELL environment variable,
allowing the expression to adapt itself to nix-shell without
triggering those adaptations when used as a dependency of another
shell.

Closes #3147

(cherry picked from commit c38c726e)

Includes documentation and test.

... in the ruby shebang test.

The test fakes the interpreter only to verify the arguments it would be
given.

This makes persistent shell environments easier to use.

Fixes #1663.

Also handle '!<output-name>' (#1694).

The nix-shell fix in 668fef2e revealed
that we had some --pure tests that incorrectly depended on PATH from
config.nix's mkDerivation being overwritten by the caller's PATH.

http://hydra.nixos.org/build/49242478

"i < argc - 1" should be "i < argc".

nix-shell -A, -p and -i are lightly tested.