Use relative imports when importing other modules in the same directory

We were using absolute imports under the assumption that the /scripts
directory is in the path. This worked in normal use because every one of our
Python scripts either were in the /scripts directory, or added the /scripts
directory to the module search path in order to reference mbedtls_dev.
However, this broke things like
```
python3 -m unittest scripts/mbedtls_dev/psa_storage.py
```

Fix this by using relative imports.

Relative imports are only supposed to be used inside a package (Python
doesn't complain, but Pylint does). So make /scripts/mbedtls_dev a proper
package by creating __init__.py.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/mbedtls_dev/__init__.py

@@ -0,0 +1,3 @@
+# This file needs to exist to make mbedtls_dev a package.
+# Among other things, this allows modules in this directory to make
+# relative impotrs.

scripts/mbedtls_dev/crypto_knowledge.py

@@ -22,7 +22,7 @@ import enum
 import re
 from typing import FrozenSet, Iterable, List, Optional, Tuple
 
-from mbedtls_dev.asymmetric_key_data import ASYMMETRIC_KEY_DATA
+from .asymmetric_key_data import ASYMMETRIC_KEY_DATA
 
 
 def short_expression(original: str, level: int = 0) -> str:

scripts/mbedtls_dev/psa_storage.py

@@ -26,7 +26,7 @@ import struct
 from typing import Dict, List, Optional, Set, Union
 import unittest
 
-from mbedtls_dev import c_build_helper
+from . import c_build_helper
 
 
 class Expr:

scripts/mbedtls_dev/test_case.py

@@ -21,7 +21,7 @@ import os
 import sys
 from typing import Iterable, List, Optional
 
-from mbedtls_dev import typing_util
+from . import typing_util
 
 def hex_string(data: bytes) -> str:
     return '"' + binascii.hexlify(data).decode('ascii') + '"'

scripts/mbedtls_dev/test_data_generation.py

@@ -29,8 +29,8 @@ import re
 from abc import ABCMeta, abstractmethod
 from typing import Callable, Dict, Iterable, Iterator, List, Type, TypeVar
 
-from mbedtls_dev import build_tree
+from . import build_tree
-from mbedtls_dev import test_case
+from . import test_case
 
 T = TypeVar('T') #pylint: disable=invalid-name