Describe platform dependencies

[mthalman]

Summary

Provides a uniform description of all platform dependencies for .NET. This is the basis for all of the other user stories in this epic. It collects the dependency data into a machine-readable format that serves as the complete database for reasoning about .NET's dependencies.

Goal

.NET contributors can describe the product's platform dependencies -- both runtime and source build toolset -- in a common format that can be used to satisfy other dependent workflows that require the data.

[mthalman]

A big question here is the granularity of the dependency. In the case of Linux, you could describe dependencies in terms of which packages are needed. However, there are some scenarios where that can be overly broad.

For example, in .NET 5.0, dotnet watch has a dependency on pgrep (see dotnet/dotnet-docker#2396). That utility is part of the procps package. In Alpine, pgrep comes pre-installed but the procps package is not. So it's not accurate to say that procps is a requirement for dotnet watch in Alpine. A more granular way would be to say that it requires just the pgrep utility and not even describe it in terms of packages.

Maybe cases where a required component comes pre-installed in the operating system do not need to be described here. It's not really actionable information when configuring what to install into an environment. Although, perhaps it is necessary from the standpoint of knowing what can be removed from an environment.

[mthalman]

Runtime platform dependency model for .NET 6 has been defined: #6170