QA: Add a set of checks for bash-completion files

Add checks for common mistakes when installing bash completion files:
legacy directory, incorrect naming, missing aliases, deprecated 'have'
function.

Changes in v2:

- silenced output (error) when sourcing completion files,

- faked _have() to enable all conditional completions in the tested
  files.
Disallow helpers in global scope in EAPI 6

Disallow calling most of the ebuild helpers in global scope since they
are meaningless in that context. Most of them are also prohibited by PMS
for all EAPIs, so EAPI 6 sounds like a good moment to finally enforce
that restriction.
Merge branch 'qa-bashcomp' into all-in-one
Merge branch 'eapi6' into all-in-one