PR #464475 enables __structuredAttrs and makes curlOpts a bash array.
Consequently, it must be extended as such to be effective.

See also https://github.com/NixOS/nixpkgs/pull/464475#issuecomment-3649765507

Allow the fetcher to authenticate with user name and password.
Note: as of now, GitLab ignores the user name (it must be set, but the
value does not matter) but this may change in the future:
https://gitlab.com/gitlab-org/gitlab/-/issues/212953

Credentials can be passed to the nix-daemon, for example, via a
read-protected `EnvironmentFile`:

```console
$ ls -l /to/secrets.txt
-rw------- 1 root root 100 Nov  1 10:42 /to/secrets.txt
```

In /to/secrets.txt:

```
 # for `fetchFromGitLab { private=true; ... }`
NIX_GITLAB_PRIVATE_USERNAME=whatever
NIX_GITLAB_PRIVATE_PASSWORD=glpat-the-access-token
 # for `fetchFromGitLab { private=true; varPrefix="EXAMPLE"; ... }`
NIX_EXAMPLE_GITLAB_PRIVATE_USERNAME=whatever
NIX_EXAMPLE_GITLAB_PRIVATE_PASSWORD=glpat-another-access-token
```

In /etc/nixos/configuration.nix:

```nix
{ config, pkgs, ... }:
{
  systemd.services.nix-daemon.serviceConfig.EnvironmentFile =
    "/to/secrets.txt";
}
```

GitLab supports HTTP Basic Authentication (credentials in `.netrc` file)
only if accessed via Git. Access via the GitLab API requires a custom
header (e.g. `PRIVATE-TOKEN`) instead. See:

* https://docs.gitlab.com/ee/user/project/settings/project_access_tokens.html#project-access-tokens
* https://docs.gitlab.com/ee/api/rest/authentication.html#personalprojectgroup-access-tokens

This patch adds `lib.repoRevToName` function that generalizes away most of the
code used for derivation name generation by `fetch*` functions (`fetchzip`,
`fetchFromGitHub`, etc, except those which are delayed until latter commits
for mass-rebuild reasons).

It's first argument controls how the resulting name will look (see below).

Since `lib` has no equivalent of Nixpkgs' `config`, this patch adds
`config.fetchedSourceNameDefault` option to Nixpkgs and then re-exposes
`lib.repoRevToName config.fetchedSourceNameDefault` expression as
`pkgs.repoRevToNameMaybe` which is then used in `fetch*` derivations.

The result is that different values of `config.fetchedSourceNameDefault` now
control how the `src` derivations produced by `fetch*` functions are to be
named, e.g.:

- `fetchedSourceNameDefault = "source"` (the default):

  ```
  $ nix-instantiate -A fuse.src
  /nix/store/<hash>-source.drv
  ```

- `fetchedSourceNameDefault = "versioned"`:

  ```
  $ nix-instantiate -A fuse.src
  /nix/store/<hash>-libfuse-2.9.9-source.drv
  ```

- `fetchedSourceNameDefault = "full"`:

  ```
  $ nix-instantiate -A fuse.src
  /nix/store/<hash>-libfuse-2.9.9-github-source.drv
  ```

See the documentation of `config.fetchedSourceNameDefault` for more info.

fetchgitlab: add sparseCheckout and forceFetchGit args

this makes the output more consistent with `fetchFromGitHub`.