style(eapi-bash): move externally accessible 'has' helper function

Makes more sense in depend.lib since isolated-functions.lib is only for
functions internal to pkgcore.
refactor(eapi-bash): define HELPER_EAPI for helpers using pkgcore-ebuild-helper

Use HELPER_EAPI variable defined in the invocation function for helpers
based on pkgcore-ebuild-helper instead of having to derive it from
HELPER_PATH when needed.
doc: convert NEWS and README files to rst
doc: misc readme updates
docs: make release header markup similar to snakeoil's
lint: cleanup print() usage

Now using logger in a few more places it makes sense and using
textwrap.dedent to format more usable multiline messages for outputting.
lint: more print() fixes
examples: use stable debug path

The debug path is always /usr/lib/debug, so don't check random ABI dirs.
pmerge: fix --preload-vdb-state help typo
eapi6: initial EAPI 6 profile
eapi6: initial get_libdir implementation

We should figure out a way to dynamically build a list of export blocks
for EAPI dependent functions, i.e. we shouldn't block exports from
eclasses for similarly named functions but instead override them with
internal PM implementations when EAPI provided functionality comes into
existence.

The current method involves explicitly adding the related functions to
the INTERNAL_FUNCS list the only current affected function being "usex"
that is defined in EAPI 5 and in the eutils.eclass for EAPI < 5. This
requires us to track implementations within eclasses which can be
problematic.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463586
eapi6: add .txz unpacking support

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=458102
eapi6: initial eapply implementation

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463768
eapi6: initial eapply_user implementation

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=475288
eapi6: initial PATCHES support in src_prepare

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463692
eapi6: mark dohtml as deprecated (not part of spec?)
eapi6: add support for econf/einstall --docdir and --htmldir (todo)

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=468202
eapi6: initial einstalldocs implementation

HTML_DOCS support might be tacked on later if that becomes part of the
spec.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=459692
eapi6: add directory support for DOCS

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=481980
eapi6: add support for explicitly disabling DOCS install

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463736