instrumentation: do not instrument main from project dependencies

This is consistent with gprbuild behavior.

Author: Jugst3r

testsuite/tests/instr-cov/126-main-in-dependency/src-main-dep/main.adb

@@ -0,0 +1,4 @@
+procedure Main is
+begin
+   null;
+end Main;

testsuite/tests/instr-cov/126-main-in-dependency/src/main_root.adb

@@ -0,0 +1,6 @@
+with Main;
+
+procedure Main_Root is
+begin
+   Main;
+end Main_Root;

testsuite/tests/instr-cov/126-main-in-dependency/test.py

@@ -0,0 +1,30 @@
+"""
+Regression testcase: checks that gnatcov correctly deals with mains in
+dependency projects. It used to crash when there was a main in one of the
+dependencies. The correct behavior should be not instrumenting them as a main.
+"""
+
+import os.path
+
+from SCOV.minicheck import build_run_and_coverage
+from SUITE.context import thistest
+from SUITE.cutils import Wdir
+from SUITE.gprutils import GPRswitches
+from SUITE.tutils import gprfor
+
+tmp = Wdir('tmp_')
+
+main_prj = gprfor(srcdirs=[os.path.join("..", "src-main-dep")],
+                  prjid="main_dep", mains=["main.adb"], objdir="obj-main")
+root_prj = gprfor(srcdirs=[os.path.join("..", "src")], deps=[main_prj],
+                  mains=["main_root.adb"], objdir="obj")
+
+# This also checks that we do not produce multiple traces
+build_run_and_coverage(
+    gprsw=GPRswitches(root_project=root_prj),
+    covlevel='stmt',
+    mains=["main_root"],
+    extra_coverage_args=["-axcov"],
+)
+
+thistest.result()

tools/gnatcov/project.adb

@@ -1431,7 +1431,8 @@ package body Project is
    --  Start of processing for Enumerate_Mains
 
    begin
-      Iterate_Projects (Root_Project, Enumerate_Mains'Access, True);
+      Iterate_Projects
+        (Root_Project, Enumerate_Mains'Access, Recursive => False);
       return Result : Main_Source_File_Array (Mains.First_Index
                                               .. Mains.Last_Index)
       do