feat: Load from .SRCINFO

This adds a function to load package info from a .SRCINFO file using python-srcinfo.

The first unit test is rather ugly, because it needs normalization on the object created by load_from_pkgbuild before comparing the result of the new function. But it shows some potential issues that will need to be solved when migrating to .SRCINFO in the whole codebase.

Related to #91

Namcap/tests/pkgbuild_test.py

@@ -14,7 +14,7 @@ import Namcap.ruleclass
 class PkgbuildTest(unittest.TestCase):
     """
     This class is the template for unit tests concerning PKGBUILD rules.
-    The setUp()	method should set self.rule to a class derived from
+    The setUp() method should set self.rule to a class derived from
     PkgbuildRule.
 
     The usual way to do that is by defining