(GH-608) Add getConventions and refactor

This commit introduces the getConventions function, which does a one-time calculation
for the changes in packaging conventions to propagate to the getArch and getOS
functions.

It updates those functions and the installer code to leverage the conventions.

Finally, it updates the test arrangement for getArch and getOS to use test cases
instead of repeating the same test over and over and adds unit tests for the new
getConventions function.

__tests__/get-conventions.test.ts

@@ -0,0 +1,57 @@
+import getConventions from "../src/get-conventions";
+
+describe('getConventions()', () => {
+  const groups = [
+    {
+      condition: 'when hugo version < 0.102.0',
+      version: '0.101.0',
+      expected: {
+        arch: {
+          darwinUniversal: false,
+          dropped32BitSupport: false,
+          standardizedNaming: false,
+        },
+        os: {
+          renamedMacOS: false,
+          downcasedAll: false
+        }
+      }
+    },
+    {
+      condition: 'when hugo version === 0.102.z',
+      version: '0.102.0',
+      expected: {
+        arch: {
+          darwinUniversal: true,
+          dropped32BitSupport: true,
+          standardizedNaming: false,
+        },
+        os: {
+          renamedMacOS: true,
+          downcasedAll: false
+        }
+      }
+    },
+    {
+      condition: 'when hugo version >= 0.103.0',
+      version: '0.103.0',
+      expected: {
+        arch: {
+          darwinUniversal: true,
+          dropped32BitSupport: true,
+          standardizedNaming: true,
+        },
+        os: {
+          renamedMacOS: true,
+          downcasedAll: true
+        }
+      }
+    }
+  ].map(function (group) {
+    return Object.assign(group, { toString: function () { return group.condition } });
+  });
+
+  test.each(groups)('%s', ({ expected, version }) => {
+    expect(getConventions(version)).toEqual(expected);
+  });
+});