Installing local package in editable mode fails for multiple indexes

[st31ny]

I have a python application with a couple of dependencies managed in a Pipfile. To test and run the application locally, I want to include it as editable installation in dev-packages. One of the application's dependencies shall be downloaded from a different index:

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[[source]]
url = "https://${GITLAB_DEPLOY_USER}:${GITLAB_DEPLOY_PASS}@<gitlab>/api/v4/groups/<group>/-/packages/pypi/simple/"
verify_ssl = true
name = "gitlab"

[packages]
requests = "*"
mydep = {index="gitlab"}

[dev-packages]
myapp = {file = ".", editable = true}
pytest = "*"

[requires]
python_version = "3.11"

However, this setup causes a dependency conflict since pip now tries to install the package from both indexes (running pipenv lock --verbose):

CRITICAL:pipenv.patched.pip._internal.resolution.resolvelib.factory:Cannot install -r /tmp/pipenv-gyt1u0np-requirements/pipenv-s87osm9z-constraints.txt (line 3) because these package versions have conflicting dependencies.
INFO:pipenv.patched.pip._internal.resolution.resolvelib.factory:
The conflict is caused by:
    myapp 3.3.1.dev137+g173fb52b.d20240930 depends on mydep
    The user requested (constraint) mydep
To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

When I set indexes = true in pyproject, credentials are apparently not passed to pip, so the download from the different index fails:

INFO:pipenv.patched.pip._internal.operations.prepare:Collecting mydep@ https://<gitlab>/api/v4/groups/<group>/-/packages/pypi/simple/mydep/ (from myapp==3.3.1.dev137+g173fb52b.d20240930->-r /tmp/pipenv-iadp574a-requirements/pipenv-tf4269xg-constraints.txt (line 5))
INFO:pipenv.patched.pip._internal.network.download:Using cached https://<gitlab>/api/v4/groups/<group>/-/packages/pypi/simple/mydep/
CRITICAL:pipenv.patched.pip._internal.utils.unpacking:Cannot unpack file /tmp/pip-unpack-t2mu_6bn/mydep (downloaded from /tmp/pip-temp-p15q7gst/mydep_5c90c7b5f8be4559887c76039c87b21e, content-type: text/html; charset=utf-8); cannot detect archive format

(Since the content-type is assumed to be text/html I assume that only an error message was downloaded).

So, how should I configure setuptools-pipfile to install myapp in editable mode? Is this possible at all?