[WIP] Use OSV design for AffectedPackages

AffectedPackges now contains all affected versions and one fix version.
This is inspired from the design documented at https://docs.google.com/document/d/1sylBGNooKtf220RHQn1I8pZRmqXZQADDQ_TOABrKTpA
Under "Format Overview", along the lines of:

    "affects": {
            "ranges": [ {
                "type": string,
                "repo": string,
                "introduced": string,
                "fixed": string
            } ],
            "versions": [ string ]
        },

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Author: Hritik14

vulnerabilities/data_source.py

@@ -42,6 +42,8 @@
 from git import Repo, DiffIndex
 from packageurl import PackageURL
 from univers.version_specifier import VersionSpecifier
+from univers.versions import BaseVersion
+from univers.versions import parse_version
 from univers.versions import version_class_by_package_type
 
 from vulnerabilities.oval_parser import OvalParser
@@ -76,29 +78,48 @@ def normalized(self):
 
 @dataclasses.dataclass(order=True, frozen=True)
 class AffectedPackage:
-    # this package MUST NOT have a version
+    # TODO: Tweak after https://github.com/nexB/univers/issues/8
+    """
+    Contains a range of affected versions and a fixed verison of a given package
+    The PackageURL supplied must *not* have a version
+    """
     package: PackageURL
-    # the version specifier contains the version scheme as is: semver:>=1,3,4
-    version_specifier: VersionSpecifier
+    affected_version_specifier: VersionSpecifier
+    fixed_version: Optional[BaseVersion]
+
+    def __post_init__(self):
+        if self.package.version:
+            raise ValueError
+
+        if self.affected_version_specifier.scheme != self.fixed_version.scheme:
+            raise ValueError
 
     def to_dict(self):
         # TODO: VersionSpecifier.__str__ is not working
         # https://github.com/nexB/univers/issues/7
         # Adjust following code when it is fixed
-        scheme = self.version_specifier.scheme
+        scheme = self.affected_version_specifier.scheme
         ranges = ",".join(
-            [f"{rng.operator}{rng.version.value}" for rng in self.version_specifier.ranges]
+            [f"{rng.operator}{rng.version.value}" for rng in self.affected_version_specifier.ranges]
         )
-        return {"package": self.package, "version_specifier": f"{scheme}:{ranges}"}
+        return {
+            "package": self.package,
+            "affected_version_specifier": f"{scheme}:{ranges}",
+            "fixed_version": self.fixed_version,
+        }
 
     @staticmethod
-    def from_dict(affected_package_dict):
-        affected_package = AffectedPackage(**affected_package_dict)
-        package = PackageURL(*affected_package.package)
-        version_specifier = VersionSpecifier.from_version_spec_string(
-            affected_package.version_specifier
+    def from_dict(aff_pkg: dict):
+        package = PackageURL(*aff_pkg["package"])
+        affected_version_specifier = VersionSpecifier.from_version_spec_string(
+            aff_pkg["affected_version_specifier"]
+        )
+        fixed_version = parse_version(aff_pkg["fixed_version"])
+        return AffectedPackage(
+            package=package,
+            affected_version_specifier=affected_version_specifier,
+            fixed_version=fixed_version,
         )
-        return AffectedPackage(package=package, version_specifier=version_specifier)
 
 
 @dataclasses.dataclass(order=True)