wrap: If the directory exists in a sub-subproject, uses it inplace

A subproject could have a sub-subproject as a git submodule, or part of
the subproject's release tarball, and still have a wrap file for it
(e.g. needed for [provide] section). In that case we need to use the
source tree for the sub-subproject inplace instead of downloading a new
copy into the main project.

This is the case with GLib 2.74, it has a subproject "gvdb" as git
submodule, and part of release tarball, it ships gvdb.wrap file as well.

mesonbuild/wrap/wrap.py

@@ -346,9 +346,12 @@ class Resolver:
         self.directory = self.wrap.directory
 
         if self.wrap.has_wrap:
-            # We have a .wrap file, source code will be placed into main
-            # project's subproject_dir even if the wrap file comes from another
-            # subproject.
+            # We have a .wrap file, use directory relative to the location of
+            # the wrap file if it exists, otherwise source code will be placed
+            # into main project's subproject_dir even if the wrap file comes
+            # from another subproject.
+            self.dirname = os.path.join(os.path.dirname(self.wrap.filename), self.wrap.directory)
+            if not os.path.exists(self.dirname):
                 self.dirname = os.path.join(self.subdir_root, self.directory)
             # Check if the wrap comes from the main project.
             main_fname = os.path.join(self.subdir_root, self.wrap.basename)

test cases/common/254 subsubproject inplace/meson.build

@@ -0,0 +1,3 @@
+project('main')
+
+subproject('sub')

test cases/common/254 subsubproject inplace/subprojects/sub/meson.build

@@ -0,0 +1,3 @@
+project('sub')
+
+subproject('subsub')

test cases/common/254 subsubproject inplace/subprojects/sub/subprojects/subsub-1.0/meson.build

@@ -0,0 +1 @@
+project('subsub')

test cases/common/254 subsubproject inplace/subprojects/sub/subprojects/subsub.wrap

@@ -0,0 +1,2 @@
+[wrap-file]
+directory = subsub-1.0