Allow use of a url and subdir in [sources]

[tecosaur]

Monorepos exist, so we shouldn't complain when somebody tries to combine the url and subdir parameters.

I think this is all that's needed to close #4026. These additions are just from looking through #3783 for suspicious omissions.

[KristofferC]

This might be all that is needed but would be good with a test to confirm.

[tecosaur]

I notice in the tests in test/sources.jl https://github.com/JuliaLang/Example.jl is used, and that actually exists. Is https://github.com/JuliaLang/ExampleMonorepo.jl needed or similar?

[fatteneder]

duplicate of #3826?

[tecosaur]

Huh, looks a bit like it. I assumed no fix was around from the state of #4026.

[tecosaur]

It would be good to get a fix into 1.11.2 if possible. If sorting out the test is the main blocker, can't we grab this for now and add a test later: it's not like it will make the current situation (not working at all) any worse.

#3826 also includes this fix, but it's marked as WIP.

[IanButterworth]

I don't think we should merge without tests of some sort.
Note that there are packages in test/test_packages. Can a test package just be added there?

The docs also need updating https://pkgdocs.julialang.org/dev/toml-files/#The-[sources]-section

[tecosaur]

That's an idea, I'll try using something from the test_packages folder and push something tonight.