common/scripts: import xbps-cycles.

From https://github.com/ahesford/xbps-cycles, that is based on
https://gist.github.com/Chocimier/de76441493ec7775c201dac0bb03ced5 .
License is compatible with void-packages. Will be run in CI, so it
should live in the same repository.

common/scripts/README.xbps-cycles.md

@@ -0,0 +1,18 @@
+# Cycle detector for void-packages
+
+This script enumerates dependencies for packages in a
+[void-packages repository](https://github.com/void-linux/void-packages)
+and identifies build-time dependency cycles.
+
+For command syntax, run `xbps-cycles.py -h`. Often, it may be sufficient to run
+`xbps-cycles.py` with no arguments. By default, the script will look for a
+repository at `$XBPS_DISTDIR`; if that variable is not defined, the current
+directory is used instead. To override this behavior, use the `-d` option to
+provide the path to your desired void-packages clone.
+
+The standard behavior will be to spawn multiple processes, one per CPU, to
+enumerate package dependencies. This is by far the most time-consuming part of
+the execution. To override the degree of parallelism, use the `-j` option.
+
+Failures should be harmless but, at this early stage, unlikely to be pretty or
+even helpful.