When operating within the framework known as "Yocto", how do I build source from within a monorepo, without storing it in the 'files' subdir? - embedded-linux

Question

I have a mono-repository ("monorepo") that contains the C++ source code for my application, as well as a tree of OE/Yocto files and directories that form the bitbake recipes required to build my final image.
I wish to build and install my application into the image, but as far as I can tell, the Yocto philosophy is that source code is "fetched" (e.g. via git), from an external place, before it is built. But in my case the source code resides in the same repository. It doesn't seem to make sense to me that the entire repository is downloaded again, by bitbake, just to fetch the source for this application, so I'm looking for a better way.
I'm familiar with the idea of putting all the source in the files/ subdirectory, alongside the recipe itself. The issue I have with that is that I don't want to keep the source in the Yocto layer's recipe tree. It can be built with the SDK, for example, or even with other completely unrelated toolchains, so it should not be buried within a Yocto layer. It has its own life, outside Yocto.
I'm also familiar with the EXTERNALSRC directive, that can be used to "point" to the source code with a relative path from the build directory. For example:
EXTERNALSRC = "${TOPDIR}/../../src/myproject"
However, this is frequently not recommended as a "production" mechanism due to path issues, and it also disables devtool:
ERROR: externalsrc is currently enabled for the myproject recipe. This prevents the normal do_patch task from working. You will need to disable this first.
So I'm looking for a recommendation on how to handle compiling an application that resides in the same repository as the recipe, without putting it in files/.
EDIT: I tried something along these lines:
SRC_URI = "file://${TOPDIR}/../../src/myproject/main.c \
file://${TOPDIR}/../../src/myproject/Makefile \
"
This did not compile, but with devtool modify myproject I was able to see that it has in fact copied the source into the build directory. The problem is that it's replicated the entire directory structure from the root all the way up to the original source directory, so my source is now sitting in a location like this:
/home/david/monorepo/yocto/build/workspace/sources/myproject/home/david/monorepo/src/myproject
do_compile will need to determine and set that working directory before it will compile.
This means that the path will change depending on the user and the location of where they've checked out the monorepo. This almost works, but doesn't seem usable as-is. Is there a way to modify where bitbake's "file" fetcher puts the source when given an absolute path?
EDIT 2:
I may have found a way that works with bitbake and devtool:
FILESEXTRAPATHS_prepend := "${TOPDIR}/../../src/myproject:"
SRC_URI = "file://main.c \
file://Makefile \
"
This seems to set up the devtool directory in a much saner way (no replication of the directory tree, just symlinks to the files in oe-local-files/* directory), and the bitbake recipe also builds and installs correctly.
Is this the right way to do it?
EDIT 3: Perhaps not, as FILESEXTRAPATHS is only intended to be modified by .bbappend recipes, not base .bb recipes - any comment on that?
Best practices dictate that you accomplish this by using FILESEXTRAPATHS from within a .bbappend file [source].
EDIT 4: PierreOlivier suggests using a relative symlink in the files/ directory to the application's source directory, so that SRC_URI can find the source as if it was actually present in files/. From what I can tell with my own experiments, this does seem to work, and devtool works with this also.
Are there any implications of this approach that I should be aware of?