Extendable Perl (#3614)

* perl: make extendable and add Module::Build package
* perl: allow 'spack create' to identify perl packages from their contents
* perl-module-build: fix indenting of package docstring
* perl: split install() method for extensions into phases
* perl: auto-detect build method (Makefile.PL vs Build.PL) and define a 'check' method
* PerlPackage: use import statements similar to those in AutotoolsPackage
* PerlModule: fix detection of Build.PL
* PerlPackageTemplate: remove extraneous lines to avoid flake8 warnings
* PerlPackageTemplate: split into separate templates for Makefile.PL and Build.PL
* PerlPackage: add cross-references to docstrings
* AutotoolsPackage: fix ambiguous cross-references to avoid errors in doc tests
* PerlbuildPackageTemplate: depend on perl-module-build if Build.PL exists