formule: add generic shared-lib methods

We have strings containing hardcoded ".dylib" extensions in homebrew-core.
To be able to bring linuxbrew-core and homebrew-core closer together,
I am introducing a new generic attribute that can be used in formulae.

Library/Homebrew/extend/os/linux/formula.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
 
 class Formula
+  undef shared_library
+
+  def shared_library(name, version = nil)
+    "#{name}.so#{"." unless version.nil?}#{version}"
+  end
+
   class << self
     undef on_linux
 

Library/Homebrew/formula.rb

@@ -1402,6 +1402,10 @@ class Formula
     ["--prefix=#{prefix}", "--libdir=#{lib}"]
   end
 
+  def shared_library(name, version = nil)
+    "#{name}.#{version}#{"." unless version.nil?}dylib"
+  end
+
   # an array of all core {Formula} names
   # @private
   def self.core_names

Library/Homebrew/test/os/linux/formula_spec.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "test/support/fixtures/testball"
 require "formula"
 
 describe Formula do
@@ -101,4 +102,12 @@ describe Formula do
       expect(f.resources.first.url).to eq("on_linux")
     end
   end
+
+  describe "#shared_library" do
+    it "generates a shared library string" do
+      f = Testball.new
+      expect(f.shared_library("foobar")).to eq("foobar.so")
+      expect(f.shared_library("foobar", 2)).to eq("foobar.so.2")
+    end
+  end
 end

Library/Homebrew/test/os/mac/formula_spec.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "test/support/fixtures/testball"
 require "formula"
 
 describe Formula do
@@ -108,4 +109,12 @@ describe Formula do
       expect(f.resources.first.url).to eq("resource_macos")
     end
   end
+
+  describe "#shared_library" do
+    it "generates a shared library string" do
+      f = Testball.new
+      expect(f.shared_library("foobar")).to eq("foobar.dylib")
+      expect(f.shared_library("foobar", 2)).to eq("foobar.2.dylib")
+    end
+  end
 end