Support absolute cache paths

[martinjlowm]

For read only systems it's common to use /tmp for caching and temporary files. This contribution enables the user to express an absolute path rather than only a relative path. The default remains the same, but simply expresses the path as absolute instead of relative.

__dirname is moved to an isolated file to avoid a circular dependency now that it's imported in config.js.

[manojmanchukonda]

Could you share the expected timeline for merging and releasing a new version?

[noverby]

@PaulDalek when will 4.0.3 be released to npm?