implicit common stanzas (a.k.a. common blocks)

It would be nice to be able to avoid writing the same imports everywhere - this becomes relevant as backpack encourages many internal libraries with similar cabal properties.

The most general way I can imagine is common import blocks that would look something like

import: my-common
library lib-a
...
library lib-b
...
test-suite test-a
...

where each piece within the indented scope implicitly imports `my-common`.

another possibility with even less boilerplate at the cost of being less general is to have toplevel implicit imports like

library-common
build-depends: base
...

so that _all_ libraries import it. There'd also be similar constructs for executable, test-suite, benchmark, foreign-library, etc.