VCS: Avoid breaking library dependency links for empty nodes

If some library doesn't contain any HDL files, no make.vlogan will
be created and the dependency tree will drop the branch that depends
on the empty library. This compacts the tree by throwing out empty
nodes and reconnect its dependencies to those that depend on the empty
library.

edalize/tools/vcs.py

@@ -40,6 +40,13 @@ class Vcs(Edatool):
     }
 
     def setup(self, edam):
+        def absorb_node(nodes, node_to_absorb):
+            for node, deps in nodes.items():
+                if node_to_absorb in deps:
+                    deps.remove(node_to_absorb)
+                    deps += nodes[node_to_absorb]
+            del nodes[node_to_absorb]
+
         super().setup(edam)
 
         incdirs = []
@@ -58,7 +65,6 @@ def setup(self, edam):
                     unused_files.remove(f)
 
         user_files = []
-        commands = {}
         libs = {}
         has_sv = False
         for f in unused_files.copy():
@@ -99,9 +105,6 @@ def setup(self, edam):
                 if not lib in libs:
                     libs[lib] = []
                 libs[lib].append((cmd, f["name"], defines))
-                if not commands.get((cmd, lib, defines)):
-                    commands[(cmd, lib, defines)] = []
-                commands[(cmd, lib, defines)].append(f["name"])
                 unused_files.remove(f)
 
         full64 = [] if self.tool_options.get("32bit") else ["-full64"]
@@ -110,6 +113,10 @@ def setup(self, edam):
         self.workdirs = []
         target_files = []
         libdeps = self.edam.get("library_dependencies", {})
+        for lib in libdeps.copy():
+            if not lib in libs:
+                absorb_node(libdeps, lib)
+
         for lib, files in libs.items():
             cmds = {}
             has_vlog = False
@@ -120,7 +127,6 @@ def setup(self, edam):
                 cmds[(cmd, defines)].append(fname)
                 if cmd == "vlogan":
                     has_vlog = True
-            commands = [["mkdir", "-p", lib]]
             i = 0
             f_files = {}
             for (cmd, defines), fnames in cmds.items():