PureConfig backend for distage-config

[neko-kai]

Unfortunately, circe-config is subject to some bugs that make it not preserve the meaning of original HOCON with 100% accuracy, e.g.

• circe-config will fail on Boolean values written as "on/true/false/off" circe/circe-config#12
• Environment variable overrides for ConfigList do not parse circe/circe-config#99

The principal reason for choosing circe-config over pureconfig in the first place was being able to use fast macros from circe-derivation to contain the penalty for compile-time codec derivation. pureconfig uses shapeless as its main derivation driver, so it's less than ideal for compile-times.
However, recently a magnolia-based deriver for pureconfig was merged: pureconfig/pureconfig#609 – but not released yet. With this change, pureconfig may be good enough to replace circe-config as a distage-config backend.

[neko-kai]

pureconfig-magnolia has been released:

https://github.com/pureconfig/pureconfig/releases/tag/v0.12.2
https://search.maven.org/artifact/com.github.pureconfig/pureconfig-magnolia_2.13

[neko-kai]

Compile-time seems to have decreased from 39s to 34s following the change, so pureconfig-magnolia is likely good enough wrt compile-time