Setup paths so different checkouts can be debugged in parallel

[alerque]

Without this it is easy to debug SILE running tests from inside the SILE path (because the current working directory is searched first by default) but it is very hard to use a version of SILE located in a directory somewhere on the system from some other path. In my case I'm trying to test different versions of SILE on book projects and need to run sile from my project's directory. This sets up the search paths in such a way that I can export SILE_PATH and have it search that for modules and packages before using the system installation path.

[alerque]

An alternative to this approach using the SILE_PATH variable which I have been using for months is found in 2fea0d9 but I never submitted that because I wasn't happy with the overhead it created, however minor, when it wasn't needed. This approach seems like a nice compromise which is useful when needed and stays out of the way when not. I just finally got tired of cherry-picking that commit into every working tree I was testing!

[coveralls]

Coverage decreased (-0.02%) to 92.4% when pulling 3003907 on alerque:debug-path into 74bf4c7 on simoncozens:master.

[alerque]

At some point I inadvertently dropped this patch from my production builds of SILE, and it just came around to bite me. I had to do some SILE debugging the other day and it refused to be run out of a checkout/build rather than being installed to the system which made the turn around time really slow. I finally came up with a fix — and it turns out if was almost character for character the same as this fix. In other words I re-invented the wheel and came up with the same design.

I think this should just become part of the core. It enables you to run un-installed versions of sile by calling the full path even if the project you are running it on is not inside the SILE directory.

[simoncozens]

Lovely, thank you!