This also adds a '--profile' option to 'nix build' (replacing 'nix-env
--set').

fix placeholder not substituted in passAsFile

Backport 'nix dev-shell' from the flakes branch

The attrError variable is no longer used but still allocated on every
call to the findAlongAttrPath function.

libexpr: remove unused attrError

This prevents users from accidentally nuking their profile via
nix-env.

(cherry picked from commit 021634e)

This is needed since we no longer produce a source tarball.

(cherry picked from commit bf70a04)

When encountering an unsupported protocol, there's no need to retry.
Chances are, it won't suddenly be supported between retry attempts;
error instead. Otherwise, you see something like the following:

    $ nix-env -i -f git://git@github.com/foo/bar
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 335 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 604 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 1340 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 2685 ms

With this change, you now see:

    $ nix-env -i -f git://git@github.com/foo/bar
    error: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1)

Don't retry on "unsupported protocol" error

This provides a pluggable mechanism for defining new fetchers. It adds
a builtin function 'fetchTree' that generalizes existing fetchers like
'fetchGit', 'fetchMercurial' and 'fetchTarball'. 'fetchTree' takes a
set of attributes, e.g.

  fetchTree {
    type = "git";
    url = "https://example.org/repo.git";
    ref = "some-branch";
    rev = "abcdef...";
  }

The existing fetchers are just wrappers around this. Note that the
input attributes to fetchTree are the same as flake input
specifications and flake lock file entries.

All fetchers share a common cache stored in
~/.cache/nix/fetcher-cache-v1.sqlite. This replaces the ad hoc caching
mechanisms in fetchGit and download.cc (e.g. ~/.cache/nix/{tarballs,git-revs*}).

This also adds support for Git worktrees (c169ea5).

This fetchers copies a plain directory (i.e. not a Git/Mercurial
repository) to the store (or does nothing if the path is already a
store path).

One use case is to pin the 'nixpkgs' flake used to build the current
NixOS system, and prevent it from being garbage-collected, via a
system registry entry like this:

  {
      "from": {
          "id": "nixpkgs",
          "type": "indirect"
      },
      "to": {
          "type": "path",
          "path": "/nix/store/rralhl3wj4rdwzjn16g7d93mibvlr521-source",
          "lastModified": 1585388205,
          "rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
      },
      "exact": true
  }

Note the fake "lastModified" and "rev" attributes that ensure that the
flake gives the same evaluation results as the corresponding
Git/GitHub inputs.

(cherry picked from commit 12f9379)

(cherry picked from commit a6ff66b)

(cherry picked from commit 78ad5b3)

(cherry picked from commit 2f49453)