Improve utils of slices (#22379)

- Move the file `compare.go` and `slice.go` to `slice.go`.
- Fix `ExistsInSlice`, it's buggy
  - It uses `sort.Search`, so it assumes that the input slice is sorted.
- It passes `func(i int) bool { return slice[i] == target })` to
`sort.Search`, that's incorrect, check the doc of `sort.Search`.
- Conbine `IsInt64InSlice(int64, []int64)` and `ExistsInSlice(string,
[]string)` to `SliceContains[T]([]T, T)`.
- Conbine `IsSliceInt64Eq([]int64, []int64)` and `IsEqualSlice([]string,
[]string)` to `SliceSortedEqual[T]([]T, T)`.
- Add `SliceEqual[T]([]T, T)` as a distinction from
`SliceSortedEqual[T]([]T, T)`.
- Redesign `RemoveIDFromList([]int64, int64) ([]int64, bool)` to
`SliceRemoveAll[T]([]T, T) []T`.
- Add `SliceContainsFunc[T]([]T, func(T) bool)` and
`SliceRemoveAllFunc[T]([]T, func(T) bool)` for general use.
- Add comments to explain why not `golang.org/x/exp/slices`.
- Add unit tests.

modules/repository/init.go

@@ -170,7 +170,7 @@ func LoadRepoConfig() {
 			}
 
 			for _, f := range customFiles {
-				if !util.IsStringInSlice(f, files, true) {
+				if !util.SliceContainsString(files, f, true) {
 					files = append(files, f)
 				}
 			}
@@ -200,12 +200,12 @@ func LoadRepoConfig() {
 	// Filter out invalid names and promote preferred licenses.
 	sortedLicenses := make([]string, 0, len(Licenses))
 	for _, name := range setting.Repository.PreferredLicenses {
-		if util.IsStringInSlice(name, Licenses, true) {
+		if util.SliceContainsString(Licenses, name, true) {
 			sortedLicenses = append(sortedLicenses, name)
 		}
 	}
 	for _, name := range Licenses {
-		if !util.IsStringInSlice(name, setting.Repository.PreferredLicenses, true) {
+		if !util.SliceContainsString(setting.Repository.PreferredLicenses, name, true) {
 			sortedLicenses = append(sortedLicenses, name)
 		}
 	}