setup_rts: fix the loading of projects installed in the toolchain prefix

Trying to resolve the project file name with GPR2.Project.Create is
wrong as this only looks up directories listed in the GPR_PROJECT_PATH
environment variable: loading projects is also supposed to use the
target information to look up projects in the toolchain installation
prefix.

As a consequence, "gnatcov instrument" fails to load the setup
configuration when it is supposed to load gnatcov_rts.gpr as installed
by "gnatcov setup" (no --prefix): GPR2 then tries to load
gnatcov_rts.gpr in the current directory, which fails.

Directly create a GPR2.Path_Name object instead with no resolution, and
let project loading do the resolution.

Note that our testing infrastructure does not allow us to install
projects in the toolchain prefix, so it is unfortunately not possible to
add an automatic regression test for this.

TN: V921-005
Change-Id: I1e720177fded95c004fcbaff9bcc5fae23ea8a14

Author: pmderodat

tools/gnatcov/setup_rts.adb

@@ -816,19 +816,35 @@ package body Setup_RTS is
       Result            : Setup_Config := Default_Setup_Config;
       Setup_Config_File : Virtual_File;
 
+      Prj_Ext      : constant String := ".gpr";
+      Prj_Filename : constant String :=
+        (if not Has_Suffix (Runtime_Project, Prj_Ext)
+            and then Exists (Runtime_Project & Prj_Ext)
+         then Runtime_Project & Prj_Ext
+         else Runtime_Project);
+      --  Name of the runtime project file to load it with GPR2.
+      --
+      --  TODO??? (V921-006) While the call to Prj.Load below is responsible
+      --  for resolving GPR file base names to full names, it is not able to
+      --  append the ".gpr" extension when missing, whereas command-line tools
+      --  are supposed to accept this case. Add ".gpr" ourselves when needed
+      --  for now, and remove this special case when GPR2 is able to do it
+
       Prj : GPR2.Project.Tree.Object;
 
       Project_File : Virtual_File;
+      --  Name of the runtime project file after it has been loaded
+      --  (i.e. resolved file name).
    begin
       --  Load the runtime project file, only to locate the prefix where it has
       --  been installed: do not print any error message, and if the project
       --  fails to load, just return the default setup config.
 
       begin
          Prj.Load
-           (Filename => GPR2.Project.Create
-                          (GPR2.Filename_Type (Runtime_Project),
-                           GPR2.Project.Default_Search_Paths (True)),
+           (Filename => GPR2.Path_Name.Create_File
+                          (GPR2.Filename_Type (Prj_Filename),
+                           GPR2.Path_Name.No_Resolution),
             Context  => GPR2.Context.Empty,
             Config   => Create_Config
                           (Target, RTS, Config_File, Runtime_Project));