Reads a Jar Manifest file or text and parses it into a set of Maps. The returned data structure has two properties: main for the main attributes, and entries containing each individual section (except for main).

Example:

            def man = readManifest file: 'target/my.jar'
            assert man.main['Version'] == '6.15.8'
            assert man.main['Application-Name'] == 'My App'
            assert man.entries['Section1']['Key1'] == 'value1-1'
            assert man.entries['Section2']['Key2'] == 'value2-2'