Parse `-r` and `-c` entries as relative to containing file (#1421)
## Summary
In a `requirements.txt` file, it turns out that the `-c` and `-r`
entries should be interpreted as relative to the file in which they're
declared, while the `-e` entries should be interpreted as relative to
the current working directory, no matter where they're defined.
Previously, we always used the current working directory; now, we use
the declaring file's directory for `-c` and `-r`.
Closes https://github.com/astral-sh/uv/issues/1367.
Closes https://github.com/astral-sh/uv/issues/1416.
