Support pip freeze URL format

`pip freeze` (https://pip.pypa.io/en/stable/cli/pip_freeze/) "outputs
installed packages in requirements format"
(https://pip.pypa.io/en/stable/reference/requirement-specifiers/).
As of pip 19.1, pip also supports specifying a requirement as a URL,
and, for packages that were installed using URL format, `pip freeze`
will output package requirements in URL format.

However, Puppet's `pip freeze` parser doesn't support URL format, so
it doesn't realize that a package is installed if it uses URL format,
causing every `puppet agent` run to attempt to reinstall the package.

This PR adds support for parsing `pip freeze` URL format.

lib/puppet/provider/package/pip.rb

@@ -101,9 +101,12 @@ def self.instances(target_command = nil)
 
   # Parse lines of output from `pip freeze`, which are structured as:
   # _package_==_version_ or _package_===_version_
+  # or _package_ @ someURL@_version_
   def self.parse(line)
     if line.chomp =~ /^([^=]+)===?([^=]+)$/
       { :ensure => Regexp.last_match(2), :name => Regexp.last_match(1), :provider => name }
+    elsif line.chomp =~ /^([^@]+) @ [^@]+@(.+)$/
+      { :ensure => Regexp.last_match(2), :name => Regexp.last_match(1), :provider => name }
     end
   end
 

spec/unit/provider/package/pip_spec.rb

@@ -38,6 +38,14 @@
       })
     end
 
+    it "should correctly parse URL format" do
+      expect(described_class.parse("real_package @ git+https://github.com/example/test.git@6b4e203b66c1de7345984882e2b13bf87c700095")).to eq({
+        :ensure   => "6b4e203b66c1de7345984882e2b13bf87c700095",
+        :name     => "real_package",
+        :provider => :pip,
+      })
+    end
+
     it "should return nil on invalid input" do
       expect(described_class.parse("foo")).to eq(nil)
     end