Detect invalid/blank/non-existent variable references in variables (e.g. *DEPEND)

[thesamesam]

The best thing to do after making a mistake is immortalise it in a bug report - because git history isn't enough!

In this commit, I fixed an error where DEPEND wasn't defined but we set:

RDEPEND="${DEPEND}" # no DEPEND in the ebuild or eclasses

We see typos like "BEPEND" every so often too. There's two types of typo:

• BEPEND="..." which is likely harder to detect without some checking of distance from known PMS-variables
• Invalid references (like this bug is for), e.g. DEPEND="${BEPEND} ..." which should be doable.