Replacing the URL of a source (e.g. PyPI) at the global level

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

Similar to one of the proposals in https://github.com/sdispater/poetry/issues/1070 (which was recently marked stale), Poetry should allow the user to override the default repository URL (PyPI). The user should be able to do this without modifying pyproject.toml.

In certain environments (e.g. corporate networks) PyPI is unavailable, but a mirror exists. These users should be able to specify the address of the mirror without modifying project files, as the mirror settings are irrelevant to contributors in different environments. Similarly, if a mirror user adds a dependency, the generated lock file should not list the user's mirror as the source. The source should remain the default (which in most cases would refer to standard PyPI).

This feature exists in pipenv, see https://github.com/pypa/pipenv/issues/2075 (where the need for this functionality is described in greater detail) and https://github.com/pypa/pipenv/pull/2281.

Start by reviewing the proposal in issue #1070 and the linked pipenv issue and pull request to understand the expected global mirror configuration. Define how the override avoids changing pyproject.toml while keeping the generated lock file's source at the default repository.