Rename GitBasedBuildpack to CustomBuildpack

Since we introduce URLs to zipped buildpacks for Diego custom buildpacks are not exclusively git based

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Make spec folders mirror source

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Accept buildpack URLs that do not end in .zip for Diego

- We used to fall back to the admin buildpacks in this case
- This means git URLs will fail while staging instead of using admin buildpacks
- [#72670838]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Avoid referring to git in buildpack url

Since Diego does not support git based buildpacks [#72670838]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>