Fix a grammar error

[wspringer]

In the existing situation, the package-decl always requires a semicolon at the end, which has some unexpected effects for nested package definitions:

Incorrectly considered valid according to the current grammar:

package foo:bar; {
  …
}

Note the semicolon between the package name and the curly braces.

Incorrectly considered invalid according to the current grammar:

package foo:bar {
  …
}

In order to fix this, this PR lifts the semicolon token out of the package-decl rule, into the wit-file rule.

[alexcrichton]

Thanks!