This will emit a warning if host customization is requested but we are
building on non-Windows (where the required APIs don't exist).

Add proper error handling and documentation

Instead, use the extra parameter to capture error information, and
throw the appropriate error from the caller of EnumResourceTypes.

This cuts the dependency from _ComputeNETCoreBuildOutputFiles on
CoreCompile. Instead, _CreateAppHost will run after compilation via
CompileDependson.

Locating the restored apphost has been factored into another target
_GetAppHostPaths, which is a dependency of both
_ComputeNETCoreBuildOutputFiles and _CreateAppHost. The destination
apphost path is also computed here, and passed to the CreateAppHost
task as an input.

This way, during a "publish --no-build", the apphost destintation
path (of the previously-generated apphost) is computed as a dependency
of _ComputeNETCoreBuildOutputFiles, and it is copied to the publish
directory without causing CoreCompile to run.

And fix a typo

The apphost is (optionally) customized by copying resources from the
managed dll, and setting the GUI bit based on the Target
property (which also modifies the managed dll). Therefore, we use the
managed dll as an input for incremental build (the apphost needs to be
updated iff the managed dll was updated). This fixes
dotnet#2554.

This also removes the unnecessary overwriteExisting flag from
AppHost.Create, fixing dotnet#2473.