plugin-getter: implement Sort interface for List

The plugin installation list should be sorted according to a name first,
then version second basis.
Right now, we only rely on the glob to add plugin installs to this list,
making the order unreliable since the lexicographical order is not the
order in which we want to see the same plugin ordered (e.g. v1.0.9 >
1.0.10).

To fix this, we implement a logic for sorting a list of installations
that does what's described above with more accuracy.

packer/plugin-getter/plugins.go

@@ -205,6 +205,54 @@ func (l InstallList) String() string {
 	return v.String()
 }
 
+// Len is the number of elements in the collection.
+func (l InstallList) Len() int {
+	return len(l)
+}
+
+var rawPluginName = regexp.MustCompile("packer-plugin-[^_]+")
+
+// Less reports whether the element with index i
+// must sort before the element with index j.
+//
+// If both Less(i, j) and Less(j, i) are false,
+// then the elements at index i and j are considered equal.
+// Sort may place equal elements in any order in the final result,
+// while Stable preserves the original input order of equal elements.
+//
+// Less must describe a transitive ordering:
+//   - if both Less(i, j) and Less(j, k) are true, then Less(i, k) must be true as well.
+//   - if both Less(i, j) and Less(j, k) are false, then Less(i, k) must be false as well.
+//
+// Note that floating-point comparison (the < operator on float32 or float64 values)
+// is not a transitive ordering when not-a-number (NaN) values are involved.
+// See Float64Slice.Less for a correct implementation for floating-point values.
+func (l InstallList) Less(i, j int) bool {
+	lowPluginPath := l[i]
+	hiPluginPath := l[j]
+
+	lowRawPluginName := rawPluginName.FindString(path.Base(lowPluginPath.BinaryPath))
+	hiRawPluginName := rawPluginName.FindString(path.Base(hiPluginPath.BinaryPath))
+
+	// We group by path, then by descending order for the versions
+	//
+	// i.e. if the path are not the same, we can return the plain
+	// lexicographic order, otherwise, we'll do a semver-conscious
+	// version comparison for sorting.
+	if lowRawPluginName != hiRawPluginName {
+		return lowRawPluginName < hiRawPluginName
+	}
+
+	return semver.Compare(lowPluginPath.Version, hiPluginPath.Version) > 0
+}
+
+// Swap swaps the elements with indexes i and j.
+func (l InstallList) Swap(i, j int) {
+	tmp := l[i]
+	l[i] = l[j]
+	l[j] = tmp
+}
+
 // Installation describes a plugin installation
 type Installation struct {
 	// Path to where binary is installed.