Remove dependencies on Hash::Merge and Clone::Choose

stow 2.3.0 added external runtime dependencies on Hash::Merge and
Clone::Choose. Historically stow hasn't had runtime dependencies other
than Perl itself, which is a useful property if you're managing the
installation of Perl using stow; the bootstrapping instructions in
stow's manual would need updating to describe how to install these two
modules (and any dependencies they have now or in the future) as well.

However, Hash::Merge is much more general than stow actually needs, so
replace the merge() call with a few lines of equivalent code -- this
avoids the external dependencies, and is clearer than the merge()
call.

Many thanks to Adam Sampson for this patch:

https://lists.gnu.org/archive/html/bug-stow/2019-06/msg00001.html

NEWS

@@ -2,6 +2,25 @@ News file for Stow.
 
 * Changes in version 2.3.1
 
+*** Remove dependencies on Hash::Merge and Clone::Choose
+
+    stow 2.3.0 added external runtime dependencies on Hash::Merge and
+    Clone::Choose. Historically stow hasn't had runtime dependencies
+    other than Perl itself, which is a useful property if you're
+    managing the installation of Perl using stow; the bootstrapping
+    instructions in stow's manual would need updating to describe how
+    to install these two modules (and any dependencies they have now
+    or in the future) as well.
+
+    However, Hash::Merge is much more general than stow actually
+    needs, so replace the merge() call with a few lines of equivalent
+    code -- this avoids the external dependencies, and is clearer than
+    the merge() call.
+
+    Many thanks to Adam Sampson for this patch!
+
+    https://lists.gnu.org/archive/html/bug-stow/2019-06/msg00001.html
+
 * Changes in version 2.3.0
 
 *** New features / changes in behaviour