Fix missing directory separator in FileDownloader

[UrGuardian4ngel]

In a project I'm working on, we have some dependencies on libraries outside of the composer ecosystem.
To do so, we are defining those external dependencies as file-typed packages.

We're currently working on migrating to composer v2.x, when I noticed an anomaly...

When installing a package with dist.type: 'file', it appears that the file gets installed in the wrong target destination?

Reproduction steps

I've created a minimal composer.json file that reproduces this issue in composer v2.x.

Sample composer.json

{
    "repositories": [
        {
            "type": "package",
            "package": {
                "name": "composer/readme-file",
                "version": "2.0.4",
                "dist": {
                    "url": "https://raw.githubusercontent.com/composer/composer/2.0.4/README.md",
                    "type": "file"
                }
            }
        }
    ],
    "require": {
        "composer/readme-file": "2.x"
    }
}

This example contains a composer/readme-file package, which pulls in the README.md file of this repository as a composer "package".

Expected behavior

As the package name is composer/readme-file, I'd expect the file to be stored at vendor/composer/readme-file/README.md.
This is also what happens with composer v1.x:

$ ./composer.phar -V
Composer version 1.10.17 2020-10-30 22:31:58

$ ./composer.phar install
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing composer/readme-file (2.0.4): Loading from cache
Writing lock file
Generating autoload files

$ tree ./vendor/
./vendor/
├── autoload.php
└── composer
    ├── ...
    └── readme-file
        └── README.md

Actual behavior

However, in composer v2.x, this is not the case anymore.

The file is stored as vendor/composer/readme-fileREADME.md (so concatenated and directly under the vendor/composer parent, as opposed to in its own vendor/composer/readme-file subdirectory):

$ ./composer.phar -V
Composer version 2.0.3 2020-10-28 15:50:55

$ ./composer.phar install
Loading composer repositories with package information
Updating dependencies
Lock file operations: 1 install, 0 updates, 0 removals
  - Locking composer/readme-file (2.0.4)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing composer/readme-file (2.0.4)
Generating autoload files

$ tree ./vendor/
./vendor/
├── autoload.php
└── composer
    ├── ...
    └── readme-fileREADME.md

[Seldaek]

Thanks