[Ada] remove extra warnings when parsing project files

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk.

It is often undesirable to have warning for unknown attributes and/or
unknown packages in project files. Warnings for unknown attributes are
no longer issued, only errors are issued for unknown attributes in
packages used by the tool. Also, in quiet mode, no warning is issued
for unknown packages.
With the following project file:

project Prj is
   package Toto is
      for Titi use "tutu";
   end Toto;
   package IDE is
      for Cleanup use "nothing";
   end IDE;
   for Externally_Built use "true";
end Prj;

the following command:
   gnatmake -P prj.gpr
should produce the following output:
   prj.gpr:2:12: warning: "toto" is not a known package name
   gnatmake: nothing to do for a main project that is externally built
and the following command:
   gnatmake -q -P prj.gpr
should produce the following output:
   gnatmake: nothing to do for a main project that is externally built

2006-10-31  Vincent Celier  <celier@adacore.com>

	* prj-dect.adb (Parse_Attribute_Declaration): Do not issue warning for
	unknown attribute in unknown package or in package that does not need
	to be checked.
	(Parse_Package_Declaration): Do not issue warning for unknown package in
	quiet output.

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