Conan registry: project-level Conan API `download_urls` returns broken URLs

Summary

The issue is in project-level Conan API. Requesting download_urls using Conan API gives URLs that are missing projects/<id> part. Getting files using returned URLs leads to 404 error. And as a result, Conan commands (e.g. conan install) fail with a message like ERROR: <package> was not found in remote '<remote>'.

After manual insertion of projects/<id> to the appropriate part of the URL, API responds with the file content without any problems.

Steps to reproduce

curl -H "Authorization: Bearer <token>" https://gitlab.example.com/api/v4/projects/<id>/packages/conan/v1/conans/<package>/<version>/<user>/<channel>/download_urls

returns the following information

{
"conan_sources.tgz":"https://gitlab.example.com/api/v4/packages/conan/v1/files/<package>/<version>/<user>/<channel>/0/export/conan_sources.tgz",
"conanfile.py":"https://gitlab.example.com/api/v4/packages/conan/v1/files/<package>/<version>/<user>/<channel>/0/export/conanfile.py",
"conanmanifest.txt":"https://gitlab.example.com/api/v4/packages/conan/v1/files/<package>/<version>/<user>/<channel>/0/export/conanmanifest.txt"
}

Example Project

What is the current bug behavior?

The prefix of URLs is https://gitlab.example.com/api/v4/packages/conan/v1/files

What is the expected correct behavior?

The prefix of URLs is https://gitlab.example.com/api/v4/projects/<id>/packages/conan/v1/files

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Ubuntu 18.04
Current User:	git
Using RVM:	no
Ruby Version:	2.6.6p146
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.9
Git Version:	2.28.0
Sidekiq Version:5.2.9
Go Version:	unknown

GitLab information
Version:	13.4.4
Revision:	e7d9e8b4224
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.9
URL:		https://gitlab.example.com
HTTP Clone URL:	https://gitlab.example.com/some-group/some-project.git
SSH Clone URL:	git@gitlab.example.com:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	13.7.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.7.0 ? ... OK (13.7.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
2/2 ... yes
3/3 ... yes
11/4 ... yes
11/5 ... yes
11/6 ... yes
11/7 ... yes
11/8 ... yes
11/9 ... yes
11/10 ... yes
11/12 ... yes
34/13 ... yes
34/14 ... yes
11/15 ... yes
14/16 ... yes
6/17 ... yes
16/18 ... yes
12/19 ... yes
11/20 ... yes
12/21 ... yes
11/22 ... yes
11/23 ... yes
8/24 ... yes
12/26 ... yes
11/27 ... yes
11/28 ... yes
11/29 ... yes
11/30 ... yes
11/31 ... yes
11/32 ... yes
11/33 ... yes
8/34 ... yes
29/35 ... yes
29/36 ... yes
11/37 ... yes
17/38 ... yes
17/39 ... yes
2/40 ... yes
17/41 ... yes
11/43 ... yes
2/44 ... yes
11/45 ... yes
11/46 ... yes
11/47 ... yes
34/49 ... yes
2/50 ... yes
8/51 ... yes
37/52 ... yes
38/53 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.24.0 ? ... yes (2.28.0)
Git user has default SSH configuration? ... yes
Active users: ... 15
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes