(PUP-2317) Make future parser issue discontinuation error for import

This makes the use of import statements raise an error.
The implementation reinstates "import" as a statement function. (This
is done by the factory when it finds a name followed by an expression
list). 
Then, to raise the error, a function written with the function API
called 'import' raises an error.

This also introduced a way to raise errors based on the issue/acceptor
system. This new error type is called Puppet::Pops::SemanticError.

It is used to issue the discontinuation error. The error itself
refers to the deprecation URL which points to the relevant documentation
for manifest directory and import.