Rename "build" directories

[dasMulli]

GitHub filters out build/ and Build/ directories for code search and go-to (press "t" on the code tab and look for files in build directories).

The dotnet/sdk repo did this by moving msbuild targets from build/ to targets/ and then changing packaging logic where the generated layout needs to have build directories.

The AspNetCore-Tooling repo and also this repo should do the same and not use directories named "build" or "Build" to be better accessible through GitHub UI.

[dougbu]

The build/ directories still exist in our release/2.1 branches but files have been migrated everywhere else. I doubt we're going to make further changes in this area.

/cc @Pilchie in case you disagree.