`poetry export -f requirements.txt` creates deprecated format URI for private git dependencies

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: OSX Catalina (python 3.7 from homebrew)
• Poetry version: 1.0.8

Issue

After exporting with poetry export -f requirements.txt --without-hashes -o foo.txt running pip install -r foo.txt gives warnings like the one below from private git dependencies.

Obtaining [REDACTED] from git+git@gitlab.com[REDACTED]

DEPRECATION: This form of VCS requirement is being deprecated: git+git@gitlab.com[REDACTED]

pip 21.0 will remove support for this functionality. A possible replacement is git+https://git@example.com/..., git+ssh://git@example.com/..., or the insecure git+git://git@example.com/.... You can find discussion regarding this at https://github.com/pypa/pip/issues/7554.

Start by reproducing poetry export -f requirements.txt --without-hashes -o foo.txt with a private git dependency, then trace the export handling for its generated VCS requirement. Done means the exported requirement uses a pip-supported git URI and pip install -r foo.txt no longer emits the deprecation warning.