Simplify Makefile.PL requirements

Requirements accumulate from one phase to the next ('configure' -> 'build'
(which also requires 'runtime' requirements) -> 'test') so there is no need
for 'test' to list requirements already listed for 'configure', 'build' or
'runtime' (or for 'build' to list requirements already listed for
'configure' or 'runtime').

(However, 'runtime' must still list everything required after installation,
but does not need to list 'perl' since 'runtime' is the phase that
'MIN_PERL_VERSION' is applied to anyway.)
Note other modules that do almost exactly the same thing

This is in response to cpan #96066.
Document that accessors are not installed if the subroutines already exist