[PPC0022] - naming convention for exceptional vs. undef-returning methods

The original PPC doc suggested two sets of "fetch an element from a meta-package" methods, named "get_..." and "can_..." to reflect the difference between methods that throw exceptions and methods that return undef when the requested entity does not exist.

my $metasym = $metapkg->get_symbol($name);  # throws if missing
my $metasym = $metapkg->can_symbol($name);  # return undef if missing

My original inspiration for can_... came from Perl's own $pkg->can(...) which returns a coderef or undef. But perhaps it's not so great.

In addition, the API shape suggested by #44 leads to an alternative form of fetching metasymbols directly, by doing things like

my $metavar = meta::variable->get('$some::package::variable');
my $metavar = meta::variable->get($pkgname, $varname);

Under that style, using ->can would not work. Perahps instead take inspiration from https://metacpan.org/pod/Object::Pad::MOP::Class#try_for_class and use get_... vs try_get_...

my $metasym = $metapkg->get_symbol($name);
my $metasym = $metapkg->try_get_symbol($name);

That also works for the constructor-style ones:

my $metavar = meta::variable->get('$some::package::variable');
my $metavar = meta::variable->try_get('$some::package::variable');

Start by reading issue #44 and the referenced Object::Pad::MOP::Class try_for_class documentation. Compare the proposed get_/can_ and get_/try_get_ naming across both object and constructor-style APIs. Done means the naming convention and its behavior are decided and recorded in the PPC documentation.