Re-enables the use of relative URLs for mod download URLs

[JordanPlayz158]

As mentioned in the commit, this should resolve #40 and https://github.com/packwiz/packwiz/issues/318, if you wish to test this, you can compile the jar via ./gradlew build and then use this command java -jar packwiz-installer-bootstrap.jar --bootstrap-no-update https://jordanplayz158.xyz/JordanPlayz158/packwiz-relative-url-example/-/raw/main/pack.toml

Any recommendations on how I could do this better, let me know!