adapted to configurable paths

[makuhama]

see getgrav/grav#3297

[mahagr]

@rhukster These changes are needed to make problems plugin to work with custom paths.

[mahagr]

I'm starting to wonder if the plugin should use those at all... As at the point when the plugin runs, all of those paths may have been changed by the configuration.

[makuhama]

I'm starting to wonder if the plugin should use those at all... As at the point when the plugin runs, all of those paths may have been changed by the configuration.

Actually that's a good point from my packager perspective. Nevertheless I would prefer to have this patch in the main branch. Sooner or later (probably after configurable paths have been documented) someone will install Grav manually and take advantage of configurable paths (as I did initially) and run into the problems plugin complaining about missing or non-writeable directories.

[mahagr]

I just updated the plugin to support this better as well as went through the code.