proc: support debugging plugins (#1414)

This change splits the BinaryInfo object into a slice of Image objects
containing information about the base executable and each loaded shared
library (note: go plugins are shared libraries).

Delve backens are supposed to call BinaryInfo.AddImage whenever they
detect that a new shared library has been loaded.

Member fields of BinaryInfo that are used to speed up access to dwarf
(Functions, packageVars, consts, etc...) remain part of BinaryInfo and
are updated to reference the correct image object. This simplifies this
change.

This approach has a few shortcomings:

1. Multiple shared libraries can define functions or globals with the
   same name and we have no way to disambiguate between them.

2. We don't have a way to handle library unloading.

Both of those affect C shared libraries much more than they affect go
plugins. Go plugins can't be unloaded at all and a lot of name
collisions are prevented by import paths.

There's only one problem that is concerning: if two plugins both import
the same package they will end up with multiple definition for the same
function.
For example if two plugins use fmt.Printf the final in-memory image
(and therefore our BinaryInfo object) will end up with two copies of
fmt.Printf at different memory addresses. If a user types
  break fmt.Printf
a breakpoint should be created at *both* locations.
Allowing this is a relatively complex change that should be done in a
different PR than this.

For this reason I consider this approach an acceptable and sustainable
stopgap.

Updates #865

service/debugger/debugger.go

@@ -1184,9 +1184,10 @@ func (d *Debugger) ListDynamicLibraries() []api.Image {
 	d.processMutex.Lock()
 	defer d.processMutex.Unlock()
 	bi := d.target.BinInfo()
-	r := make([]api.Image, len(bi.Images))
-	for i := range bi.Images {
-		r[i] = api.ConvertImage(bi.Images[i])
+	r := make([]api.Image, 0, len(bi.Images)-1)
+	// skips the first image because it's the executable file
+	for i := range bi.Images[1:] {
+		r = append(r, api.ConvertImage(bi.Images[i]))
 	}
 	return r
 }