Also reuse subproject-based fallback dependencies

This allows a project to use the same fallbacks dependency from the same
subproject multiple times in the same way that external dependencies can be.

Also change the format of the dependency identifier to ensure that fallback
checks with different dirname/varname aren't mistakenly reused. We now use
a tuple for this because the format is simpler to construct and it gives us the
same immutability guarantees as a string which is needed for using it as
a dictionary key.

0 files changed, 0 insertions, 0 deletions