Refactor Package structure

This commit cleans up the package structure, to make two new top-level
packages `internal` (for internal-use packages) and `pkg` (for general
shared utility code).

pkg/fsx/same.go

@@ -0,0 +1,87 @@
+package fsx
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// SameFile checks if path1 and path2 refer to the same file.
+// If both files exist, they are compared using [os.SameFile].
+// If both files do not exist, the paths are first compared syntactically and then via recursion on [filepath.Dir].
+func SameFile(path1, path2 string) bool {
+
+	// initial attempt: check if directly
+	same, certain := couldBeSameFile(path1, path2)
+	if certain {
+		return same
+	}
+
+	// second attempt: find the directory names and base paths
+	d1, n1 := filepath.Dir(path1), filepath.Base(path1)
+	d2, n2 := filepath.Dir(path2), filepath.Base(path2)
+
+	// if we have different file names (and they don't exist)
+	// we don't need to continue
+	if n1 != n2 {
+		return false
+	}
+
+	// compare the base names!
+	{
+		same, _ := couldBeSameFile(d1, d2)
+		return same
+	}
+}
+
+// couldBeSameFile checks if path1 might be the same as path2.
+//
+// If both files exist, compares using [os.SameFile].
+// Otherwise compares absolute paths using string comparison.
+//
+// same indicates if they might be the same file.
+// authorative indiciates if the result is authorative.
+func couldBeSameFile(path1, path2 string) (same, authorative bool) {
+	{
+		// stat both files
+		info1, err1 := os.Stat(path1)
+		info2, err2 := os.Stat(path2)
+
+		// both files exist => check using os.SameFile
+		// the result is always authorative
+		if err1 == nil && err2 == nil {
+			same = os.SameFile(info1, info2)
+			authorative = true
+			return
+		}
+
+		// only 1 file errored => they could be different
+		if (err1 == nil) != (err2 == nil) {
+			return
+		}
+
+		// only 1 file does not exist => they could be different
+		if os.IsNotExist(err1) != os.IsNotExist(err2) {
+			return
+		}
+	}
+
+	{
+		// resolve paths absolutely
+		rpath1, err1 := filepath.Abs(path1)
+		rpath2, err2 := filepath.Abs(path2)
+
+		// if either path could not be resolved absolutely
+		// fallback to just using clean!
+		if err1 != nil {
+			rpath1 = filepath.Clean(path1)
+		}
+		if err2 != nil {
+			rpath2 = filepath.Clean(path2)
+		}
+
+		// compare using strings
+		same = rpath1 == rpath2
+		authorative = same // positive result is authorative!
+		return
+	}
+}