Search paths DOTNET_HOME in host

[TheRealPiotrP]

We need to add two more default paths to the host:

1. zip/layout
   If running dotnet from build artifacts or from zip, the DOTNET_HOME directory for tools in the CLI will be in ..\runtime...This is a very common scenario so the path should be considered.
2. When a deps file is present it should include a pointer to the runtime [does today if runtime package is referenced]. Host should take this into account when looking for Runtime.

[blackdwarf]

@anurse is this part of the corehost spec?

[analogrelay]

Not currently. I'm not a huge fan of it as-is. My concern is that it assumes a lot about the layout of files or adds path information to the deps file, which are both things I'm not a big fan of :).

The tools in DOTNET_HOME\bin already have an embedded runtime, so they don't need to use ..\runtime\coreclr to find it. Also, dotnet run ensures DOTNET_HOME is set properly before launching the app (there was a bug in that, but it should be fixed now). Finally, there will be OS-specific defaults for the installed version of the runtime. These should mean that only us devs or people doing more complicated things will need to set DOTNET_HOME, and I think we can handle it :)

[analogrelay]

DOTNET_HOME is going away (/cc @schellap @davidfowl) so this is irrelevant