[oe] Unpacking nested git repos in recipe?

[Ole Wolf]

I want to create a recipe that includes two git repositories. Fetching them
works as I'd expect when I specify their sources as follows:

SRC_URI = " \
    git://github.com/boostorg/boost.git;name=boost \
    git://github.com/boostorg/build.git;name=build \
"
SRCREV_boost = "42d94a72ccbc64e2f4652a69c2d292b1028da087"
SRCREV_build = "c2574b2d5a481f88864d1e75ce775f41cbc03bad"

However, I can't figure out how to unpack them properly. The first
repository, "boost," is to be unpacked into the default ".../git" directory
whereas the other repository, "build," is to be unpacked into a
subdirectory of "boost." I had hoped that the git fetching might provide a
"subdir" modifier, but it seems that's not supported.

So, somehow I need to be able to state that "boost" must be unpacked first,
followed by "build," which is to be unpacked into ".../git/tools/build."

Is this possible without resorting to a custom do_unpack function?
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