Packit does not support absolute file:/// paths for dependency-mapping

[paulchambers]

Dependency mapping should allow users to use a local copy of a dependency by specifying an absolute file:/// path to a file mounted into the build. Instead packit always appends a base path to the path specified in the mapping

Expected Behavior

A dependency-mapping with the path failed to fetch dependency: failed to open file: file:///platform/bindings/dependency-mapping/binaries/yarn_1.22.19_linux_noarch_bionic_3235ba55.tgz should look in the given directory

Current Behavior

The packit based yarn buildpack instead gives this error:

failed to fetch dependency: failed to open file: open /cnb/buildpacks/paketo-buildpacks_yarn/1.3.7/platform/bindings/dependency-mapping/binaries/yarn_1.22.19_linux_noarch_bionic_3235ba55.tgz: no such file or directory

Possible Solution

Don't append the base path if the mapping starts with file:/// (a triple slash) (This is the behavior used in libpak based buildpacks)

This comment https://github.com/paketo-buildpacks/packit/blob/d558b87c97641ff5acfe9270248ecdfaaa4e7226/postal/service.go#L244C1-L247C1 suggests there may be buildpacks that expect the current behavior so it may need to append the base path for a non-absolute file URI (eg a double slash)

Steps to Reproduce

build a container using a dependency-mapping with an absolute file path

Motivations

This bug prevented using a local copy of the yarn dependency when deps.paketo.io failed.