feature(eapi-bash): support generic command deprecation for EAPIs

This works similarly to the already existing banned command support.
Briefly explained, within the helpers directory a symlink is created
inside the related EAPI directory that points to the
internals/deprecated script and is named for the deprecated command.
This script then overrides the real command and when called during the
processing of an ebuild with a matching EAPI echoes a QA warning before
executing the intended command.
feature(eapi6): mark dohtml as deprecated
fix(eapi-bash): make colored messages work as expected

Colors were being disabled on import of isolated-functions.lib file (I'm
not sure why) and variables relating to color escape codes weren't being
exported so things like eqawarn messages weren't getting colored
properly in certain cases, e.g. when run via deprecated commands such as
dohtml in EAPI 6.