packages.md: document depends build targets

Summary:
Introduced in D5608.

Backport of core [[bitcoin/bitcoin#16691 | PR16691]].
Only commit 2483266
(bitcoin/bitcoin@2483266)
is applicable for us, since the first one is autotools documentation
while we recommand cmake.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5634

depends/packages.md

@@ -181,3 +181,19 @@ For us, it's much easier to just link a static `libsecondary` into a shared
 static or dynamic `libsecondary`, that's not our concern. With a static
 `libsecondary`, when we need to link `libprimary` into our executable, there's
 no dependency chain to worry about as `libprimary` has all the symbols.
+
+## Build targets:
+
+To build an individual package (useful for debugging), the following build
+targets are available.
+
+    make ${package}
+    make ${package}_fetched
+    make ${package}_extracted
+    make ${package}_preprocessed
+    make ${package}_configured
+    make ${package}_built
+    make ${package}_staged
+    make ${package}_postprocessed
+    make ${package}_cached
+    make ${package}_cached_checksum