wire: use go/packages for analysis (google/go-cloud#623)

Unfortunately, this does come with a ~4x slowdown to Wire, as it is
now pulling source for all transitively depended packages, but not
trimming comments or function bodies. This is due to limitations with
the ParseFile callback in go/packages.

This comes with a single semantic change: when performing analysis, Wire
will now evaluate everything with the wireinject build tag. I updated
the build tags tests accordingly. Prior to this PR, only the packages
directly named by the package patterns would be evaluated with the
wireinject build tag. Dependencies would not have the wireinject build
tag applied. There isn't a way to selectively apply build tags in go/packages,
and there isn't a clear benefit to applying it selectively. Being consistent with
other Go tooling provides greater benefit.

I deleted the vendoring test, as non-top-level vendoring
becomes obsolete with modules.

go/packages now parses comments by default, so now the generated code
includes comments for non-injector declarations.

Fixes google/go-cloud#78

internal/wire/analyze.go

@@ -44,11 +44,11 @@ type call struct {
 	// out is the type this step produces.
 	out types.Type
 
-	// importPath and name identify the provider to call for kind ==
+	// pkg and name identify the provider to call for kind ==
 	// funcProviderCall or the type to construct for kind ==
 	// structProvider.
-	importPath string
-	name       string
+	pkg  *types.Package
+	name string
 
 	// args is a list of arguments to call the provider with.  Each element is:
 	// a) one of the givens (args[i] < len(given)), or
@@ -199,7 +199,7 @@ dfs:
 			}
 			calls = append(calls, call{
 				kind:       kind,
-				importPath: p.ImportPath,
+				pkg:        p.Pkg,
 				name:       p.Name,
 				args:       args,
 				fieldNames: p.Fields,
@@ -419,7 +419,7 @@ func verifyAcyclic(providerMap *typeutil.Map, hasher typeutil.Hasher) []error {
 						fmt.Fprintf(sb, "cycle for %s:\n", types.TypeString(a, nil))
 						for j := i; j < len(curr); j++ {
 							p := providerMap.At(curr[j]).(*ProvidedType).Provider()
-							fmt.Fprintf(sb, "%s (%s.%s) ->\n", types.TypeString(curr[j], nil), p.ImportPath, p.Name)
+							fmt.Fprintf(sb, "%s (%s.%s) ->\n", types.TypeString(curr[j], nil), p.Pkg.Path(), p.Name)
 						}
 						fmt.Fprintf(sb, "%s\n", types.TypeString(a, nil))
 						ec.add(errors.New(sb.String()))