NuGet Sources Command output has changed

I was trying out the NuGet Aliases and found that `HasSource` was not working. After some investigation and running the command `nuget sources list` manually I can see that the output format

```
Registered Sources:

1. nuget.org [Enabled]
https://api.nuget.org/v3/index.json
2. Microsoft Visual Studio Offline Packages [Enabled]
C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\
```

has changed and is not was is expected by the alias (L#142)

https://github.com/cake-build/cake/blob/3dccfa1dec00c17b6c038d2564a99cc36d03598a/src/Cake.Common/Tools/NuGet/Sources/NuGetSources.cs#L141-L142

A naive fix that was backwards compatable would be something like

```c#
process => result = process.GetStandardOutput().Any(line => line.TrimStart() == source || line.Contains(source))
```

but this has the problem of partial matches if one source name is a substring of another. (How likely?)

Alternatively we could have some regex that matches on the line format

```c#
process => result = process.GetStandardOutput().Any(line => line.TrimStart() == source || Regex.IsMatch(line, $@"\d+\. {source} \["))
```

second method is my preference and happy to put in a pr for either or alternate solution

**Update 1:** using `NuGet Version: 4.1.0.2450`

Start in src/Cake.Common/Tools/NuGet/Sources/NuGetSources.cs around lines 141-142 and run `nuget sources list` to compare its current output with the alias's matching logic. Define completion as HasSource recognizing the numbered source format without partial-name matches, while preserving compatibility with the previously expected output.

Written by the indexing model from the issue text.