switch should clear last scratch save

[ceedubs]

Currently it's easy to run into unexpected behavior if you use the switch command after having saved a scratch file. For example:

• Save a scratch file with some new definitions.
• switch to another project/namespace.
• Run add or update
• Now you have added definitions to the second project that might have dependencies that are only in the first project.

Another example:

• Save a main function in a scratch file.
• switch to a second project.
• run main, expecting to run the main function saved in project 2.
• Instead you run the main function from your scratch file (targeting project 1), that might not even be added to the codebase.

The same should apply to any command that changes your namespace. Examples that come to mind: clone, upgrade, ns/cd.