Support for Cake-Build Projects

Edit from the Dependabot team: This is the main tracking issue for adding support for Cake.

PR:
https://github.com/dependabot/dependabot-core/pull/958

Related Issues:
https://github.com/dependabot/dependabot-core/issues/733

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It would be great if dependabot could support updating dependencies which are defined by people using Cake-Build (https://cakebuild.net/).

Cake is a build automation framework, that allows people to take a dependency on NuGet packages that are either an addin, tool or module. You can see examples of the types of dependency definitions here:

https://github.com/cake-contrib/Cake.Recipe/blob/develop/Cake.Recipe/Content/addins.cake#L5-L23
https://github.com/cake-contrib/Cake.Recipe/blob/develop/Cake.Recipe/Content/tools.cake#L5-L20
https://github.com/chocolatey/ChocolateyGUI/blob/develop/setup.cake#L1
https://github.com/chocolatey/ChocolateyGUI/blob/develop/setup.cake#L2

Notice, that there are different ways for these dependencies to be expressed, based on how the project is created.

Cake itself is also a dependency, which is normally defined in the `tools/packages.config` files, as shown here:

https://github.com/cake-contrib/Cake.Recipe/blob/develop/tools/packages.config

This is just a normal NuGet packages.config file, so this might already be supported in dependabot today.

We already have an attempt at doing something similar to this on a much smaller scale. For example this PR:

https://github.com/cake-contrib/Cake.Recipe/pull/263

Was created by our automated process.

Documentation on the pre-processor directives that can define these dependencies can be found here:

https://cakebuild.net/docs/fundamentals/preprocessor-directives

./cc @greysteil

Start by reviewing PR 958, the Cake.Recipe dependency examples, and Cake's preprocessor-directive documentation. Compare the addin, tool, module, and tools/packages.config forms to determine the supported dependency cases and how completion should be verified.

Written by the indexing model from the issue text.