Fixes mirror bug when originalHost is excluded

[TisVictress]

Also adds tests to makes sure that url path is complete

[sophiewigmore]

what does this mean?

[sophiewigmore]

Can you elaborate on what the bug that you're solving is?

[TisVictress]

The user can use {originalHost} as a placeholder to include the original url path when specifying a mirror. For example, if the original url is https://artifacts.paketo.io/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz and the mirror is https://user:pass@local-mirror.example.com/buildpacks-dependencies/{originalHost}, then this will translate to https://user:pass@local-mirror.example.com/buildpacks-dependencies/artifacts.paketo.io/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz. But if the placeholder is excluded, then the mirror translates to https://user:pass@local-mirror.example.com/buildpacks-dependencies/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz

[TisVictress]

@sophiewigmore In the initial change, the mirror url was not properly constructed and did not include the actual dependency archive. For instance, with the following:
original: https://artifacts.paketo.io/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz
mirror: https://user:pass@local-mirror.example.com/buildpacks-dependencies
...it would not append the actual tgz file to be downloaded. The result instead was https://user:pass@local-mirror.example.com/buildpacks-dependencies/artifacts.paketo.io
This also happened with special hostnames:
original: https://github.com/nginx/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz
mirror: https://mirror.example.org/public-github
...the result would incorrectly be https://mirror.example.org/public-github when it should be https://mirror.example.org/public-github/nginx/nginx_1.24.0_linux_x64_jammy_fa1dfe46.tgz

The initial change only worked correctly if the {originalHost} placeholder was used. This is due to line 35 of dependency_mirror.go. The path needed to be moved outside of the substitution.

I also updated the tests because the test urls did not include an appropriate scheme and path

[sophiewigmore]

This is helpful info! thank you