Include core formulae from API in Formula.all.

This is needed to allow `Formula.all` to behave as expected when using
the API. Let's also (immediately, because it's broken) deprecate the
`CoreTap#formula_files` method.

Fixes #15122

Library/Homebrew/formula.rb

@@ -1827,12 +1827,6 @@ class Formula
     CoreTap.instance.formula_names
   end
 
-  # an array of all core {Formula} files
-  # @private
-  def self.core_files
-    CoreTap.instance.formula_files
-  end
-
   # an array of all tap {Formula} names
   # @private
   def self.tap_names
@@ -1851,12 +1845,6 @@ class Formula
     @names ||= (core_names + tap_names.map { |name| name.split("/").last }).uniq.sort
   end
 
-  # an array of all {Formula} files
-  # @private
-  def self.files
-    @files ||= core_files + tap_files
-  end
-
   # an array of all {Formula} names, which the tap formulae have the fully-qualified name
   # @private
   def self.full_names
@@ -1872,11 +1860,11 @@ class Formula
       odeprecated "Formula#all without --all or HOMEBREW_EVAL_ALL"
     end
 
-    files.map do |file|
-      Formulary.factory(file)
+    (core_names + tap_files).map do |name_or_file|
+      Formulary.factory(name_or_file)
     rescue FormulaUnavailableError, FormulaUnreadableError => e
       # Don't let one broken formula break commands. But do complain.
-      onoe "Failed to import: #{file}"
+      onoe "Failed to import: #{name_or_file}"
       $stderr.puts e
 
       nil

Library/Homebrew/tap.rb

@@ -514,11 +514,17 @@ class Tap
   def self.formula_files_by_name(tap)
     cache_key = "formula_files_by_name_#{tap}"
     cache.fetch(cache_key) do |key|
-      cache[key] = tap.formula_files.each_with_object({}) do |file, hash|
-        # If there's more than one file with the same basename: intentionally
-        # ignore the later ones here.
-        hash[file.basename.to_s] ||= file
-      end
+      cache[key] = tap.formula_files_by_name
+    end
+  end
+
+  # @private
+  sig { returns(T::Hash[String, Pathname]) }
+  def formula_files_by_name
+    formula_files.each_with_object({}) do |file, hash|
+      # If there's more than one file with the same basename: intentionally
+      # ignore the later ones here.
+      hash[file.basename.to_s] ||= file
     end
   end
 
@@ -1011,6 +1017,14 @@ class CoreTap < Tap
     Homebrew::API::Formula.all_aliases.keys
   end
 
+  # @private
+  sig { returns(T::Array[Pathname]) }
+  def formula_files
+    return super if Homebrew::EnvConfig.no_install_from_api? || installed?
+
+    raise TapUnavailableError, name
+  end
+
   # @private
   sig { returns(T::Array[String]) }
   def formula_names
@@ -1018,6 +1032,27 @@ class CoreTap < Tap
 
     Homebrew::API::Formula.all_formulae.keys
   end
+
+  # @private
+  sig { returns(T::Hash[String, Pathname]) }
+  def formula_files_by_name
+    return super if Homebrew::EnvConfig.no_install_from_api?
+
+    formula_names.each_with_object({}) do |name, hash|
+      # If there's more than one file with the same basename: intentionally
+      # ignore the later ones here.
+      hash[name] ||= sharded_formula_path(name)
+    end
+  end
+
+  private
+
+  # @private
+  sig { params(name: String).returns(Pathname) }
+  def sharded_formula_path(name)
+    # TODO: add sharding logic.
+    formula_dir/"#{name}.rb"
+  end
 end
 
 # Permanent configuration per {Tap} using `git-config(1)`.