[Ada] Refactor handling of project path

[Arnaud Charlet <charlet at adacore dot com>]

This type encapsulates the manipulation of project files, in particular
finding projects and caching the results. The cache is now stored in
this type, rather than being a global table shared by all loaded trees.
This is in preparation for aggregate projects, that can redefine their
own project path.

The code to manipulate project path was moved from prj-ext to prj-env
for circular dependency reasons (the new type is needed in prj-tree).

Tested on x86_64-pc-linux-gnu, committed on trunk

2010-10-05  Emmanuel Briot  <briot@adacore.com>

	* gnatcmd.adb, prj-proc.adb, prj-part.adb, prj-ext.adb, prj-ext.ads,
	switch-m.adb, clean.adb, prj-nmsc.adb, prj-nmsc.ads, prj-env.adb,
	prj-env.ads, prj-tree.adb, prj-tree.ads (Project_Search_Path): New type.

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