Fix fetch of credentials when cache is seeded with username

[zanieb]

Fixes the failure to lookup credentials in #3205

The issue is that we seed the cache with the index URL which includes a username but no password. We did not ensure that a password was present in the cached credentials before attempting a request with them. Now, the cache will not return credentials when a username is provided and the cached credentials have no password — the cached credentials are useless in that case.

Tested with a Google Artifact Registry and keyring

RUST_LOG=uv=trace cargo run -q -- pip install requests --index-url https://[email protected]/<project>/pypi/simple/ --no-cache --keyring-provider subprocess -v

[jenshnielsen]

@zanieb I can confirm that this restores the behaviour of 0.1.35 for Azure artifacts.

[BurntSushi]

I buy what you're selling.