Use HelpLink for errors/warnings that already have a URL

We have several errors/warnings that have a URL in the text of the message. After #5488 (thanks, @jmezac!) we'll have a structured way to represent this that will eventually (https://github.com/dotnet/project-system/issues/6335) be used in the Visual Studio UI.

We should use that!

Likely candidates:

```sh-session
$ rg "fwlink|aka\.ms" -g *resx
src\Tasks\Resources\Strings.resx
683: MSB3152: To enable 'Download prerequisites from the same location as my application' in the Prerequisites dialog box, you must download file '{0}' for item '{1}' to your local machine. For more information, see http://go.microsoft.com/fwlink/?LinkId=616018.
1568: MSB3276: Found conflicts between different versions of the same dependent assembly. Please set the "AutoGenerateBindingRedirects" property to true in the project file. For more information, see http://go.microsoft.com/fwlink/?LinkId=294190.
1971: MSB3474: The task "{0}" is not supported on the .NET Core version of MSBuild. Use the Microsoft XML Serializer Generator package instead. See https://go.microsoft.com/fwlink/?linkid=858594 for more information.
2156: MSB3644: The reference assemblies for {0} were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks
2545: MSB3783: Project "{0}" depends upon SDK "{1} v{2}" which was released originally for apps targeting "{3} {4}". To verify whether "{1} v{2}" is compatible with "{5} {6}", contact the SDK author or see http://go.microsoft.com/fwlink/?LinkID=309181.
2553: MSB3842: Project "{0}" depends upon SDK "{1} v{2}" which supports apps targeting "{3} {4}". To verify whether "{1} v{2}" is compatible with "{5} {6}", contact the SDK author or see http://go.microsoft.com/fwlink/?LinkID=309181.
2730: MSB4803: The task "{0}" is not supported on the .NET Core version of MSBuild. Please use the .NET Framework version of MSBuild. See https://aka.ms/msbuild/MSB4803 for further details.

src\MSBuild\Resources\Strings.resx
791: For more detailed information, see https://aka.ms/msbuild/docs

src\Build\Resources\Strings.resx
1197: Project file contains ToolsVersion="{0}". This toolset may be unknown or missing, in which case you may be able to resolve this by installing the appropriate version of MSBuild, or the build may have been forced to a particular ToolsVersion for policy reasons. Treating the project as if it had ToolsVersion="{1}". For more information, please see http://go.microsoft.com/fwlink/?LinkId=293424.
```