Make --namespace-packages the default

Unfortunately, a number of tests fail. Please help fixing them :-)
Especially the ones to do with cache inconsistencies... There's at least
one known bug here: #7276

Author: hauntsaninja

mypy/main.py

@@ -444,7 +444,7 @@ def add_invertible_flag(flag: str,
         title='Import discovery',
         description="Configure how imports are discovered and followed.")
     add_invertible_flag(
-        '--namespace-packages', default=False,
+        '--no-namespace-packages', dest="namespace_packages", default=True,
         help="Support namespace packages (PEP 420, __init__.py-less)",
         group=imports_group)
     imports_group.add_argument(

mypy/options.py

@@ -87,7 +87,7 @@ def __init__(self) -> None:
         # Intended to be used for disabling specific stubs.
         self.follow_imports_for_stubs = False
         # PEP 420 namespace packages
-        self.namespace_packages = False
+        self.namespace_packages = True
 
         # disallow_any options
         self.disallow_any_generics = False

test-data/unit/check-modules.test

@@ -2617,12 +2617,13 @@ from foo.bar import x
 x = 0
 
 [case testClassicNotPackage]
+# flags: --no-namespace-packages
 from foo.bar import x
 [file foo/bar.py]
 x = 0
 [out]
-main:1: error: Cannot find implementation or library stub for module named 'foo.bar'
-main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+main:2: error: Cannot find implementation or library stub for module named 'foo.bar'
+main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
 
 [case testNamespacePackage]
 # flags: --namespace-packages

test-data/unit/fine-grained-modules.test

@@ -845,7 +845,7 @@ main:2: error: Argument 1 to "f" has incompatible type "int"; expected "str"
 ==
 main:1: error: Cannot find implementation or library stub for module named 'p.a'
 main:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-main:1: error: Cannot find implementation or library stub for module named 'p'
+main:2: error: "object" has no attribute "a"
 
 [case testDeletePackage2]
 import p

test-data/unit/semanal-errors.test

@@ -316,15 +316,16 @@ x = y
 tmp/k.py:2: error: Name 'y' is not defined
 
 [case testPackageWithoutInitFile]
+# flags: --no-namespace-packages
 import typing
 import m.n
 m.n.x
 [file m/n.py]
 x = 1
 [out]
-main:2: error: Cannot find implementation or library stub for module named 'm.n'
-main:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
-main:2: error: Cannot find implementation or library stub for module named 'm'
+main:3: error: Cannot find implementation or library stub for module named 'm.n'
+main:3: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+main:3: error: Cannot find implementation or library stub for module named 'm'
 
 [case testBreakOutsideLoop]
 break