emit require module name err hint only when valid

emit require module name err hint only when valid

RT #131098

The helpful "you may need to install" hint which 'require' sometimes
includes in its error message these days (split across multiple lines for
clarity):

    $ perl -e'require Foo::Bar'
    Can't locate Foo/Bar.pm in @INC
        (you may need to install the Foo::Bar module)
        (@INC contains: ... ) at ...

is a bit over-enthusiastic when the pathname hasn't actually been derived
from a module name:

    $ perl -e'require "Foo.+/%#Bar.pm"'
    Can't locate Foo.+%#Bar.pm in @INC
        (you may need to install the Foo.+::%#Bar module)
        (@INC contains: ... ) at ...

This commit changes things so that the hint message is only emitted if the
reverse-mapped module name is legal as a bareword:

    $ perl -e'require "Foo.+/%#Bar.pm"'
    Can't locate Foo.+%#Bar.pm in @INC
        (@INC contains: ... ) at ...