keg: don't rely on #to_formula

It doesn't always work.
For example, a keg could have been installed with a formula from a
URL, which Homebrew now does not know how to access.

Fixes #1496.

Library/Homebrew/keg.rb

@@ -109,13 +109,20 @@ class Keg
     end
 
     keg_names = kegs.map(&:name)
-    kegs_by_name = kegs.group_by(&:to_formula)
+    kegs_by_source = kegs.group_by { |k| [k.name, Tab.for_keg(k).tap] }
+
     remaining_formulae.each do |dependent|
       required = dependent.missing_dependencies(hide: keg_names)
-      required.select! { |f| kegs_by_name.key?(f) }
-      next unless required.any?
 
-      required_kegs = required.map { |f| kegs_by_name[f].sort_by(&:version).last }
+      required_kegs = required.map do |f|
+        f_kegs = kegs_by_source[[f.name, f.tap]]
+        next unless f_kegs
+
+        f_kegs.sort_by(&:version).last
+      end
+
+      next unless required_kegs.any?
+
       return required_kegs, [dependent.to_s]
     end
 

Library/Homebrew/test/test_keg.rb

@@ -317,12 +317,16 @@ end
 
 class InstalledDependantsTests < LinkTests
   def stub_formula_name(name)
-    stub_formula_loader formula(name) { url "foo-1.0" }
+    f = formula(name) { url "foo-1.0" }
+    stub_formula_loader f
+    stub_formula_loader f, "homebrew/core/#{f}"
+    f
   end
 
   def setup_test_keg(name, version)
-    stub_formula_name(name)
+    f = stub_formula_name(name)
     keg = super
+    Tab.create(f, DevelopmentTools.default_compiler, :libcxx).write
     Formula.clear_cache
     keg
   end
@@ -345,6 +349,17 @@ class InstalledDependantsTests < LinkTests
     end
   end
 
+  # Test with a keg whose formula isn't known.
+  # This can happen if e.g. a formula is installed
+  # from a file path or URL.
+  def test_unknown_formula
+    Formulary.unstub(:loader_for)
+    dependencies []
+    alter_tab { |t| t.source["path"] = nil }
+    assert_empty @keg.installed_dependents
+    assert_nil Keg.find_some_installed_dependents([@keg])
+  end
+
   def test_no_dependencies_anywhere
     dependencies nil
     assert_empty @keg.installed_dependents