Refactor graph representation (uber-go#301)

This is the first PR to prepare Dig for in-graph modifications. 

This PR contains several changes to allow further extensibility in the
graph representation used in Dig, and decouple reflection Type info
from the graph algorithms as much as possible.

Specifically, this PR makes the following changes:

- Add `internal/graph` package which contains Dig-agnostic graph
  algorithm used for cycle detection
- Remove the somewhat duplicated embedded cycle detection logic in
  Container and cycle.go
- Add `graphHolder` type in `Container` which holds all the graph nodes
  and implements the graph iterator interface used for graph algorithm
  in `internal/graph`.
- Add `paramGroupedSlice` as part of node in graph.
- Remove the param visitor funcs and related helpers.
- Rename `node` to `constructorNode`.

There is also a change in the way cycle error messages are displayed.
Previously, the errors looked like:

```
typeA provided by funcName (file_location.go:line_number)
  depends on typeB provided by funcName (file_location.go:line_number)
  depends on typeC provided by funcName (file_location.go:line_number)
 ... and so on
```

This error message generation depended on knowing the "edge" info
between nodes, which is the reflect.Type that connects the two
constructor nodes. 

Now that the graph representation holds not only constructor nodes but
also value groups, and because graph representation does not know what
type holds the nodes together, this needs to be changed.

Now, the error messages look like:
```

func(*typeB) *typeA provided by funcName(file_location.go:line_number)
  depends on func(*typeC) *typeB provided by funcName(file_location.go:line_number)
  depends on func(*typeA) *typeC provided by funcName(file_location.go:line_number)
... and so on
```

(Note that the function signature is now printed instead of the type
that holds the nodes as dependencies.)

For param value groups, the error message skips over it so that the
error message doesn't look cluttered but that can be subject to change
in the future.

Co-authored-by: Abhinav Gupta <[email protected]>

Author: sywhang

cycle.go

@@ -0,0 +1,63 @@
+// Copyright (c) 2019 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package dig
+
+import (
+	"bytes"
+	"fmt"
+
+	"go.uber.org/dig/internal/digreflect"
+)
+
+type cycleErrPathEntry struct {
+	Key  key
+	Func *digreflect.Func
+}
+
+type errCycleDetected struct {
+	Path []cycleErrPathEntry
+}
+
+func (e errCycleDetected) Error() string {
+	// We get something like,
+	//
+	//   func(*bar) *foo provided by "path/to/package".NewFoo (path/to/file.go:42)
+	//   	depends on func(*baz) *bar provided by "another/package".NewBar (somefile.go:1)
+	//   	depends on func(*foo) baz provided by "somepackage".NewBar (anotherfile.go:2)
+	//   	depends on func(*bar) *foo provided by "path/to/package".NewFoo (path/to/file.go:42)
+	//
+	b := new(bytes.Buffer)
+
+	for i, entry := range e.Path {
+		if i > 0 {
+			b.WriteString("\n\tdepends on ")
+		}
+		fmt.Fprintf(b, "%v provided by %v", entry.Key, entry.Func)
+	}
+	return b.String()
+}
+
+// IsCycleDetected returns a boolean as to whether the provided error indicates
+// a cycle was detected in the container graph.
+func IsCycleDetected(err error) bool {
+	_, ok := RootCause(err).(errCycleDetected)
+	return ok
+}