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/error.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+#
+# QAPI error classes
+#
+# Copyright (c) 2017-2019 Red Hat Inc.
+#
+# Authors:
+#  Markus Armbruster <armbru@redhat.com>
+#  Marc-André Lureau <marcandre.lureau@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2.
+# See the COPYING file in the top-level directory.
+
+
+class QAPIError(Exception):
+    def __init__(self, info, col, msg):
+        Exception.__init__(self)
+        self.info = info
+        self.col = col
+        self.msg = msg
+
+    def __str__(self):
+        loc = str(self.info)
+        if self.col is not None:
+            assert self.info.line is not None
+            loc += ':%s' % self.col
+        return loc + ': ' + self.msg
+
+
+class QAPIParseError(QAPIError):
+    def __init__(self, parser, msg):
+        col = 1
+        for ch in parser.src[parser.line_pos:parser.pos]:
+            if ch == '\t':
+                col = (col + 7) % 8 + 1
+            else:
+                col += 1
+        QAPIError.__init__(self, parser.info, col, msg)
+
+
+class QAPISemError(QAPIError):
+    def __init__(self, info, msg):
+        QAPIError.__init__(self, info, None, msg)