remove redundant code to calculate test/build prereqs
convert versions into a format more suitable for EUMM

Older EUMM versions complain about dotted decimal versions with v
prefixes.  Without the v prefix, they will at least pass the versions
through more properly to MYMETA and Makefile files, so other parts of
the toolchain can handle them.  Remove the v prefix for multi-dot
versions, and convert single dot versions to their numified form.
test EUMM version normalization
note the version drop in [MakeMaker]
remove unused lib includes
lower required version of Module::Build

0.3601 is only required for sharedirs; 0.28 is when install_base changed incompatibly.
v5.020

        - the default required version for ExtUtils::MakeMaker in [MakeMaker]
          has been removed
        - load DateTime lazily
        - the default required version for Module::Build in [ModuleBuild] has
          been lowered
create Build.PL, Makefile.PL in filegatherer phase

This allows other plugins to not just see that this file exists (at early
phases such as file munging, and generate files accordingly -- e.g.
[InstallGuide]), but allow other plugins to add content earlier (e.g.
Devel::CheckLib). The template will be evaluated at the normal time, once the
prereq list is finalized.