keg: handle dependencies of moved/renamed formulae

In #1497 I switched from Keg#to_formula for comparing kegs to formulae
to comparing the name and tap in the keg's tab to the name and tap of
the formula.

However, this fails to match if the name and tap of the formula have
changed since the keg was installed, so it's clearly better to use
Keg#to_formula where possible, and fall back to the information in the
tab when #to_formula can't be used.

Library/Homebrew/keg.rb

@@ -126,7 +126,18 @@ class Keg
     end
 
     keg_names = kegs.map(&:name)
-    kegs_by_source = kegs.group_by { |k| [k.name, Tab.for_keg(k).tap] }
+    kegs_by_source = kegs.group_by do |keg|
+      begin
+        # First, attempt to resolve the keg to a formula
+        # to get up-to-date name and tap information.
+        f = keg.to_formula
+        [f.name, f.tap]
+      rescue FormulaUnavailableError
+        # If the formula for the keg can't be found,
+        # fall back to the information in the tab.
+        [keg.name, Tab.for_keg(keg).tap]
+      end
+    end
 
     remaining_formulae.each do |dependent|
       required = dependent.missing_dependencies(hide: keg_names)

Library/Homebrew/test/keg_test.rb

@@ -373,6 +373,7 @@ class InstalledDependantsTests < LinkTestCase
       t.source["tap"] = "some/tap"
       t.source["path"] = nil
     end
+
     dependencies [{ "full_name" => "some/tap/foo", "version" => "1.0" }]
     assert_equal [@dependent], @keg.installed_dependents
     assert_equal [[@keg], ["bar 1.0"]], Keg.find_some_installed_dependents([@keg])
@@ -388,7 +389,7 @@ class InstalledDependantsTests < LinkTestCase
     Formula["bar"].class.depends_on "baz"
 
     result = Keg.find_some_installed_dependents([@keg, @tap_dep])
-    assert_equal [[@tap_dep], ["bar"]], result
+    assert_equal [[@keg, @tap_dep], ["bar"]], result
   end
 
   def test_no_dependencies_anywhere
@@ -411,6 +412,23 @@ class InstalledDependantsTests < LinkTestCase
     assert_nil Keg.find_some_installed_dependents([@keg, @dependent])
   end
 
+  def test_renamed_dependency
+    dependencies nil
+
+    stub_formula_loader Formula["foo"], "homebrew/core/foo-old"
+    renamed_path = HOMEBREW_CELLAR/"foo-old"
+    (HOMEBREW_CELLAR/"foo").rename(renamed_path)
+    renamed_keg = Keg.new(renamed_path.join("1.0"))
+
+    Formula["bar"].class.depends_on "foo"
+
+    result = Keg.find_some_installed_dependents([renamed_keg])
+    assert_equal [[renamed_keg], ["bar"]], result
+  ensure
+    # Move it back to where it was so it'll be cleaned up.
+    (HOMEBREW_CELLAR/"foo-old").rename(HOMEBREW_CELLAR/"foo")
+  end
+
   def test_empty_dependencies_in_tab
     dependencies []
     assert_empty @keg.installed_dependents