Adds simplest Removable check

This will also eventually need some satisfiability love.

installed.go

@@ -2,7 +2,9 @@ package pm
 
 import (
 	"errors"
+	"fmt"
 	"sort"
+	"strings"
 )
 
 // Installed tracks installed packages.
@@ -28,5 +30,44 @@ func (i Installed) Traverse() <-chan Meta {
 
 // Removable calculates if the packages requested in "in" can all be removed.
 func (i Installed) Removable(names []string) (Metas, error) {
-	return nil, errors.New("NYI")
+	inm := map[Name]bool{}
+
+	// XXX (sm): here we simply check if the package exists; eventually we'll
+	// have to check transitive dependencies, and deal with explicitly and
+	// implicitly installed packages.
+
+	found := map[Name]Meta{}
+	for _, name := range names {
+		n := Name(name)
+		inm[n] = true
+		if m, ok := i[n]; ok {
+			found[n] = m
+		}
+	}
+
+	if len(found) > len(inm) {
+		return nil, errors.New("should not have been able to find more than asked for, but did; internals are inconsistent.")
+	} else if len(inm) > len(found) {
+		// user asked for something that isn't installed.
+		missing := []string{}
+		for _, name := range names {
+			if _, ok := found[Name(name)]; !ok {
+				missing = append(missing, name)
+			}
+		}
+		return nil, fmt.Errorf("packages not installed: %v", strings.Join(missing, ", "))
+	}
+
+	if len(found) != len(inm) {
+		return nil, fmt.Errorf("escapes logic")
+	}
+
+	// XXX (sm): the ordering here will also eventually depend on transitive
+	// dependencies.
+	r := Metas{}
+	for _, m := range found {
+		r = append(r, m)
+	}
+
+	return r, nil
 }