Extend TargetSelector with a TargetAllPackages constructor with the
corresponding concrete command line syntax 'all'. The interpretation of
this is extended for all commands to be the same as if the list of all
packages local to the project were given.

Since the concrete syntax for the meta-target 'all' can in principle
clash with a package 'all' or a component 'all', this short form of
syntax is made to be ambigious with the existing short forms for
packages or components, and new more qualified forms are added.

This means that a user writing 'cabal build all' in a project where
there is a local package 'all' or a component 'all' in the package in
the cwd (but not any package 'all' as a dependency or any component
'all' in any package other than the one in the cwd), will be informed
that the syntax is ambigious and will be told the more qualified forms
of the possible targets. These would be ':all' and 'pkg:all'. The use of
the usual ':' separator with an empty string is the explicit indicator
of a meta namespace. Since 'pkg:all' itself is also potentially
ambigious with a package named 'pkg' containing a component 'all' then
there is a further qualified form ':pkg:all' to select the package named
'all'.

This more qualified syntax need only be used in these highly unusual
ambigious cases and the user will be informed. The intention, as has
been the case previously, is to allow a normal syntax that is short and
convenient but potentially ambigious in rare cases, while still allowing
all cases to be expressed unambigiously.