Fix regressions in the Python and npm package host
Created by: olafurpg
Previously, the Python and npm package hosts failed to download packages because the download URLs were empty due to a regression that was introduced in the PR https://github.com/sourcegraph/sourcegraph/pull/38074
This PR fixes the regression by populating the download URLs before sending HTTP requests to those URLs.
Test plan
Run full integration tests to verify that the re-enabled test is no longer failing.