Add proof-of-concept for downloading and initialising kdb-common libraries from GitHub

[jasraj]

[jasraj]

Includes breaking changes to 'require.q' - the removal of .require.location.root the most likely to cause problems.

Example usage:

q) .rrg.get "gh://jasraj/kdb-systemd:latest"
...
2021.11.20 16:28:24.147 INFO  pid-416 jas 0 [./src/require.q:.require.addPath(93):1] Adding new path to 'require' search path [ Path: :/home/jas/git/kdb-common/.require-remote-deps/kdb-systemd ] [ Rescan: yes ]
2021.11.20 16:28:24.147 DEBUG pid-416 jas 0 [./src/require.q:.require.rescanRoot(85):1] Rescanning all require library paths [ Paths: :/home/jas/git/kdb-common, :/home/jas/git/kdb-common/.require-remote-deps/kdb-systemd ]
...
q) .require.lib`sdi
2021.11.20 16:28:26.956 INFO  pid-416 jas 0 [./src/require.q:.require.i.load(128):1] Loading library: sdi
2021.11.20 16:28:26.956 INFO  pid-416 jas 0 [./src/require.q:.require.i.load@(138):1] Loading /home/jas/git/kdb-common/.require-remote-deps/kdb-systemd/src/sdi.q
...