Support a platform-dependent dependency mapping

[nebhale]

Currently, online packaged buildpacks must download their contributed dependency from the open internet. There are situations where the URIs for those download locations are blocked from within a corporate network. libpack's DependencyCache should noticed the existence of an artifact or artifacts in /platform that defines a mapping between a buildpack's dependency (id/version) and a different URL that it should be downloaded from. This should happen transparently (save for a different log message) to both the buildpack author using DependencyCache and the application developer using a buildpack that uses DependencyCache.

[nebhale]

ConfigMap with entry-per-buildpack, TOML array per dependency.

[[mapping]]
id = "jdk"
version = "8.0.242"
uri = "https://internal:8080/jdk8.tar.gz
[[mapping]]
id = "jdk"
version = "11.0.6"
uri = "https://internal:8080/jdk11.tar.gz
[[mapping]]
id = "jre"
version = "8.0.242"
uri = "https://internal:8080/jre8.tar.gz
[[mapping]]
id = "jre"
version = "11.0.6"
uri = "https://internal:8080/jre11.tar.gz

[nebhale]

@ekcasey You're looking for DependencyResolver and you'll want a single log message when you detect this directory. A single line mentioning that you're using platform overrides for downloading, and then the URIs will reflect that in their normal log message.