Translate `build-tool-depends` to `BDEPEND`

[hololeap]

Currently, hackport overlooks build-tool-depends fields in Cabal files, which could lead to missing dependencies. The dependencies fall under "programs that need to be executed during the build" as mentioned here.

An example is hw-kafka-client:

  build-tool-depends:   hspec-discover:hspec-discover

This does not get added at all to the ebuild. A proper output might look like:

BDEPEND="test? (
	dev-haskell/hspec-discover
)"