name—a string designator; not evaluated.
package—the package named by name.
Causes the the package named by name
to become the current package—that is, the value of *package*.
If no such package already exists, an error of type package-error is signaled.
Everything in-package does is also performed at compile time
if the call appears as a top level form.
The variable *package* is assigned.
If the in-package form is a top level form,
this assignment also occurs at compile time.
An error of type package-error is signaled if the specified package does not exist.