The prebuilt dependencies are specific to the client app (not the
server).
This also avoids to reference the parent directory (../) from
app/meson.build.
This aims to fix two issues with the previous implementation:
1. the whole content of downloaded archives were extracted, while only
few files are necessary;
2. the archives were extracted in the prebuild-deps/ directory as is.
As a consequence of (2), the actual directory name relied on the root
directory of the archive. For adb, this root directory was always
"platform-tools", so when bumping the adb version, the target directory
already existed and the dependency was not upgraded (the old one had to
be removed manually).
Expose common function to download a file and check its checksum, but
let the custom script for each dependency extract only the needed files
and reorganize the content if necessary.
