qapi: Split up scripts/qapi/common.py

The QAPI code generator clocks in at some 3100 SLOC in 8 source files.
Almost 60% of the code is in qapi/common.py.  Split it into more
focused modules:

* Move QAPISchemaPragma and QAPISourceInfo to qapi/source.py.

* Move QAPIError and its sub-classes to qapi/error.py.

* Move QAPISchemaParser and QAPIDoc to parser.py.  Use the opportunity
  to put QAPISchemaParser first.

* Move check_expr() & friends to qapi/expr.py.  Use the opportunity to
  put the code into a more sensible order.

* Move QAPISchema & friends to qapi/schema.py

* Move QAPIGen and its sub-classes, ifcontext,
  QAPISchemaModularCVisitor, and QAPISchemaModularCVisitor to qapi/gen.py

* Delete camel_case(), it's unused since commit e98859a9b9 "qapi:
  Clean up after recent conversions to QAPISchemaVisitor"

A number of helper functions remain in qapi/common.py.  I considered
moving the code generator helpers to qapi/gen.py, but decided not to.
Perhaps we should rewrite them as methods of QAPIGen some day.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191018074345.24034-7-armbru@redhat.com>
[Add "# -*- coding: utf-8 -*-" lines]

scripts/qapi/doc.py

@@ -7,7 +7,8 @@
 
 from __future__ import print_function
 import re
-import qapi.common
+from qapi.gen import QAPIGenDoc, QAPISchemaVisitor
+
 
 MSG_FMT = """
 @deftypefn {type} {{}} {name}
@@ -216,10 +217,10 @@ def texi_entity(doc, what, ifcond, base=None, variants=None,
             + texi_sections(doc, ifcond))
 
 
-class QAPISchemaGenDocVisitor(qapi.common.QAPISchemaVisitor):
+class QAPISchemaGenDocVisitor(QAPISchemaVisitor):
     def __init__(self, prefix):
         self._prefix = prefix
-        self._gen = qapi.common.QAPIGenDoc(self._prefix + 'qapi-doc.texi')
+        self._gen = QAPIGenDoc(self._prefix + 'qapi-doc.texi')
         self.cur_doc = None
 
     def write(self, output_dir):