refactor the build.gradle file, extract publish tasks to separated gradle file

Former-commit-id: 539b8d16917a8a60e07a7f1466ce9ad0c84ca94e

dependency-check-gradle/conf/publish/gradlePluginsPortal.gradle

@@ -0,0 +1,14 @@
+// publish to gradle plugin portal
+pluginBundle {
+    website = 'https://github.com/wmaintw/DependencyCheck'
+    vcsUrl = 'git@github.com:wmaintw/DependencyCheck.git'
+    description = 'This is dependency check gradle plugin.'
+    tags = ['dependency check', 'security']
+
+    plugins {
+        dependencyCheckPlugin {
+            id = 'dependency.check'
+            displayName = 'dependency-check'
+        }
+    }
+}