refactor: prepare to decouple the "model migration" package and "models" package (#38533)

Migrations should never use model structs directly, because the model
structs can be different in different releases. e.g. if one migration uses
"User" model, it works in the early releases, then one day, when the
User model changes, the migration breaks because it will use the
new (incorrect) User model, it should only use the old User model.

The same to "modules/structs".

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

modelmigration/fixtures/Test_UpdateOpenMilestoneCounts/expected_milestone.yml

@@ -0,0 +1,19 @@
+# type Milestone struct {
+#   ID              int64                  `xorm:"pk autoincr"`
+#   IsClosed        bool
+#   NumIssues       int
+#   NumClosedIssues int
+#   Completeness    int  // Percentage(1-100).
+# }
+-
+  id: 1
+  is_closed: false
+  num_issues: 3
+  num_closed_issues: 1
+  completeness: 33
+-
+  id: 2
+  is_closed: true
+  num_issues: 5
+  num_closed_issues: 5
+  completeness: 100

modelmigration/fixtures/Test_UpdateOpenMilestoneCounts/issue.yml

@@ -0,0 +1,25 @@
+# type Issue struct {
+#   ID               int64 `xorm:"pk autoincr"`
+#   RepoID           int64 `xorm:"INDEX UNIQUE(repo_index)"`
+#   Index            int64 `xorm:"UNIQUE(repo_index)"` // Index in one repository.
+#   MilestoneID      int64 `xorm:"INDEX"`
+#   IsClosed         bool  `xorm:"INDEX"`
+# }
+-
+  id: 1
+  repo_id: 1
+  index: 1
+  milestone_id: 1
+  is_closed: false
+-
+  id: 2
+  repo_id: 1
+  index: 2
+  milestone_id: 1
+  is_closed: true
+-
+  id: 4
+  repo_id: 1
+  index: 3
+  milestone_id: 1
+  is_closed: false

modelmigration/fixtures/Test_UpdateOpenMilestoneCounts/milestone.yml

@@ -0,0 +1,19 @@
+# type Milestone struct {
+#   ID              int64                  `xorm:"pk autoincr"`
+#   IsClosed        bool
+#   NumIssues       int
+#   NumClosedIssues int
+#   Completeness    int  // Percentage(1-100).
+# }
+-
+  id: 1
+  is_closed: false
+  num_issues: 4
+  num_closed_issues: 2
+  completeness: 50
+-
+  id: 2
+  is_closed: true
+  num_issues: 5
+  num_closed_issues: 5
+  completeness: 100