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
test all WriteMakefile arguments
MIN_PERL_VERSION is harmless; leave it

This bit of code was added in commit 5c6a1202 but there doesn't seem to be any
need for it there.  We can still add a 'use' line, while keeping this data --
which also is added to MYMETA.*.
also get the minimum perl requirement from configure and test prereqs
also fold configure-requires prereqs into %FallbackPrereqs