Clean target should not read assets file, issues with /t:Clean;Build

[nguerrera]

Currently, the Clean target ends up triggering a read of the assets file if it exists.

This is due to the following competing concerns:

1. Assets file resolution is responsible for populating transitive project references
2. Transitive project references must be added before AssignProjectConfiguration
3. Clean must run AssignProjectConfiguration in order to clean referenced projects

We don't need the transitive project references to clean, but the problem is that once AssignProjectConfiguration is run, it won't re-run if there's a subsequent invocation of the Build task.

We have some code to special case clean, and it is smart enough to not kick in if the build is triggered by /t:rebuild, but not if /t:build;clean is used.

Issues with /t:Clean;Build

• When there has not been a prior restore, it will not trigger "assets file is missing, run restore", but instead resolve no assets and move on to compilaton that will fail to resolve anything.
• It will hide all assets log messages

So to be clear, this issue tracks two related things:

1. /t:Clean;Build should work the same as /t:Rebuild and /t:Clean in all cases.
2. /t:Clean alone should never read the assets file