feat: support path query parameter (sub-directory) even Unity 2019.2 …

…or earlier

Detail information: https://forum.unity.com/threads/some-feedback-on-package-manager-git-support.743345/
  * path must be a relative path to the root of the repository.
  * path query parameter **must** be placed **before** the revision anchor. The reverse order will fail.
  * A package manifest (package.json) is expected in the specified path.
  * e.g. With Path query parameter: `https://github.com/user/repo.git?path=/example/folder`
  * e.g. With revision anchor and path query parameter: `https://github.com/user/repo.git?path=/example/folder#v1.2.3`

Packages/com.coffee.git-dependency-resolver/Editor/PackageMeta.cs

@@ -16,6 +16,7 @@ internal class PackageMeta
             new Regex(
                 @"^(git\+)?" +
                 @"(?<url>[^#?]*)" +
+                @"(\?(?<query>[^#]*))?" +
                 @"(#(?<rev>.*))?",
                 k_RegOption);
 
@@ -29,13 +30,15 @@ internal class PackageMeta
         internal SemVersion version { get; private set; }
         public string rev { get; private set; }
         public string url { get; private set; }
+        public string path { get; private set; }
         public PackageMeta[] dependencies { get; private set; }
 
         private PackageMeta()
         {
             name = "";
             url = "";
             rev = "";
+            path = "";
             version = new SemVersion(0);
             dependencies = new PackageMeta [0];
         }
@@ -114,12 +117,15 @@ public static PackageMeta FromNameAndUrl(string name, string url)
             // Get version from revision/branch/tag
             package.SetVersion(package.rev);
 
+            package.ProcessUrlQuery(m.Groups["query"].Value);
             return package;
         }
 
         public string GetPackagePath(string clonePath)
         {
-            return clonePath;
+            return string.IsNullOrEmpty(path)
+                ? clonePath
+                : (clonePath + "/" + path).Replace("//", "/");
         }
 
         private void SetVersion(string ver)
@@ -129,6 +135,28 @@ private void SetVersion(string ver)
                 version = v;
         }
 
+        private void ProcessUrlQuery(string urlQuery)
+        {
+            // Process url query.
+            var queries = urlQuery.Split('&')
+                .Select(q => q.Split('='))
+                .Where(q => q.Length == 2)
+                .ToDictionary(q => q[0], q => q[1]);
+
+            // path query.
+            string value;
+            if (queries.TryGetValue("path", out value))
+                path = value.Trim('/');
+
+            // version query.
+            if (queries.TryGetValue("version", out value))
+            {
+                SemVersion v;
+                SemVersion.TryParse(value, out v);
+                version = v;
+            }
+        }
+
         public IEnumerable<PackageMeta> GetAllDependencies()
         {
             return dependencies;
@@ -141,7 +169,7 @@ public string GetDirectoryName()
 
         public override string ToString()
         {
-            return string.Format("{0}@{1} ({2})", name, version, rev);
+            return string.Format("{0}@{1} ({2}) [{3}]", name, version, rev, path);
         }
     }
 }