Make builtin envVariables more explicit

[yunxing]

Injecting built-in envVariables makes it harder to debug. We should make those variables more explicit for people to debug.

The general approach I'm thinking is to add a prefix to all built-in variables. For example, instead of "sandbox", we can use "__dependencyEnv_sandbox" instead. This can also reduce the chance of collision with other user-defined variables.

[jordwalke]

Good idea. We need a shorter prefix name like yo though. Also, we can have a "debug" command - that explains why environment variables were set the way they are, whether or not they come from transitive dependencies, and which negotiations took place in order to construct them that way.