This patch helps CPerl mode to detect various declarations which should
go into the imenu index but were missed, and also prevents some false
positives from being included.  Undetected package declarations also led
to wrong namespace attributions for subroutines declared within those
packages.

This comes with a new approach to handle Perl syntax: CPerl mode suffers
from a lot of very complex ad-hoc regular expressions, not always
consistent with each other, and very difficult to maintain.  Instead, I
have created some regular expressions for basic syntactic building
blocks which I hope to use in upcoming refactorings to fix some
inaccuracies, and in particular to add support for "modern" or even
upcoming Perl syntax.

The regular expressions are built with rx syntax, but with the
restricted set of rx features which is available in Emacs 26.1 (as
eventually this should be the minimum version for an ELPA release).

These are the constructs which are now correctly processed:

  # really, really weird namespaces
  package ::; # yes, that's perfectly valid!

  # package with a version (since Perl 5.14)
  package Foo::Bar 1.23;

  # package with a block (since Perl 5.14)
  package Foo::Bar {
      ...;
  }

  # subroutines with my, state, our (since Perl 5.18)
  my sub lexical_sub { ...; }
  state sub also_lexical { ...; }
  our sub shared_in_scope { ...; }

  # subroutines in a "foreign" namespace (since "forever")
  sub Elsewhere::routine { ...; }

  # subroutines without space before the attribute (since "forever")
  sub attr:lvalue { ...; } # this is sub attr with attribute :lvalue

-- 
Cheers,
haj