Future EAPI/DEPENDENCIES variable

Since we are considering adding new dependency types, few developers are suggesting we should adopt exheres-0 single DEPENDENCIES variable.

A copy-paste from Exheres for smarties:

DEPENDENCIES="       build+run:            foo/bar        run:            foo/baz        post:            foo/monkey    "

And:

DEPENDENCIES="       blah? ( cat/build-and-run )        foo? (            cat/build-and-run            build:                cat/build-only            run:                cat/run-only                bar? (                    cat/run-only                    post:                        cat/post-only                )                cat/run-only-again        )        cat/build-and-run"

Shortly saying:
 * foo: starts a block of dependencies of type foo,
 * foo+bar+baz: means that the listed package belong to foo, bar and baz-type dependencies at the same time,
 * the default block is build+run,
 * blocks are scoped to the enclosing braces.

Advantages:
 * easier way of specifying common dependencies in really weird cases,
 * ability to replace one-letter-DEPEND variables with a long, human-readable labels.

Disadvantages:
 * necessity of rewriting whole *DEPEND when migrating to the new EAPI,
 * necessity of handling two different dependency syntaxes in eclasses,
 * added complexity to the dependency syntax parser (and it is spooky already),
 * necessity of converting the new block to old-style dependencies -- for compatibility with external tools,
 * reinventing the wheel -- introducing a new syntax to avoid proper use of bash variables.