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/source.py

@@ -0,0 +1,67 @@
+#
+# QAPI frontend source file info
+#
+# Copyright (c) 2019 Red Hat Inc.
+#
+# Authors:
+#  Markus Armbruster <armbru@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2.
+# See the COPYING file in the top-level directory.
+
+import copy
+import sys
+
+
+class QAPISchemaPragma(object):
+    def __init__(self):
+        # Are documentation comments required?
+        self.doc_required = False
+        # Whitelist of commands allowed to return a non-dictionary
+        self.returns_whitelist = []
+        # Whitelist of entities allowed to violate case conventions
+        self.name_case_whitelist = []
+
+
+class QAPISourceInfo(object):
+    def __init__(self, fname, line, parent):
+        self.fname = fname
+        self.line = line
+        self.parent = parent
+        self.pragma = parent.pragma if parent else QAPISchemaPragma()
+        self.defn_meta = None
+        self.defn_name = None
+
+    def set_defn(self, meta, name):
+        self.defn_meta = meta
+        self.defn_name = name
+
+    def next_line(self):
+        info = copy.copy(self)
+        info.line += 1
+        return info
+
+    def loc(self):
+        if self.fname is None:
+            return sys.argv[0]
+        ret = self.fname
+        if self.line is not None:
+            ret += ':%d' % self.line
+        return ret
+
+    def in_defn(self):
+        if self.defn_name:
+            return "%s: In %s '%s':\n" % (self.fname,
+                                          self.defn_meta, self.defn_name)
+        return ''
+
+    def include_path(self):
+        ret = ''
+        parent = self.parent
+        while parent:
+            ret = 'In file included from %s:\n' % parent.loc() + ret
+            parent = parent.parent
+        return ret
+
+    def __str__(self):
+        return self.include_path() + self.in_defn() + self.loc()