Pre-process missing build-requires

[kallisti5]

haikuporter will prompt over and over for each missing dependency as it is requested:

(haikuporter qemu)
build-requires "devel:libfdt" of package "qemu-3.1.0" could not be resolved
(install devel:libfdt)

(haikuporter qemu)
build-requires "devel:libcapstone" of package "qemu-3.1.0" could not be resolved
(install devel:libcapstone)

User experience would be better if haikuporter presented the full list of missing requirements to the user. (depth 1, it's fair missing dependencies of dependencies would result in multiple prompts)

[kallisti5]

(a workaround would be using --get-dependencies in this example)