(RFC) New standard package: GNU Info

[Michael Orlitzky]

On https://trac.sagemath.org/ticket/32242 I propose adding the
standalone GNU "info" reader as a new standard package.

In sage/interfaces/singular.py, we currently manually parse Singular's
Info file to obtain docstrings for Singular's functions. This is not
great:

1. We're maintaining an ad-hoc parser for a standard format that can
already be read reliably by another tool that everyone has.

2. The manual parser needs to know the location of Singular's info
file. This is a headache for packagers, because everyone needs to
hard-code their own path, and it's one of the things preventing us
from using a system copy of Singular. Using GNU Info alleviates 
the problem, because in essence we can just run "info singular" 
and count on the documentation being installed in the right place.

3. The manual parser is looking for Singular's info file relative to
an environment variable named SINGULARPATH, which isn't used
correctly at the moment (it's an upstream variable that we mangle
in sage-env, but can be multi-valued). This is more of a boring
technical problem, but also has to be solved before we can use
a system copy of Singular. Using GNU Info means we don't care
about the variable at all and can delete it.

GNU Info (or its superproject, Texinfo) is already installed on most
linux distributions and is available everywhere, although you may wind
up building the SPKG on BSD. But ideally, the spkg-configure.m4 will
find "info" in everyone's $PATH and this additional dependency will be
a no-op.

[Martin R]

See also https://trac.sagemath.org/ticket/21734