crate_universe: Enable modext isolation.

[criemen]

This should allow us to build our python and ruby
code independently - in particular, we can now do shallow checkouts of one without the other.
Previously, the modext introduced a cross-dependency. This also reduces the amount of work we do in the
crate universe processing for the other language, even though it's unused.

This also gives us a speed bump of about 4min on all python-only jobs, as the main slowness of rules_rust is due to our ruby git dependencies, which we now don't pull in any longer.

The new file paths look like this:

rules_rust~~_crate~ql~~r~rd__tree-sitter-embedded-template-0.21.0
rules_rust~~_crate~ql~~cp~py_deps__tree-sitter-graph-0.7.0

[criemen]

Marking as draft, as this PR requires renaming the codeql module to ql, and shortening cargo_ruby to c, and ruby_deps to rd, to yield the same path lengths. Anything longer than that triggers long path errors with cl.exe on Windows 😬

I still think this is worth it, but this requires a bit more discussion than just enabling modext isolation.
The reason isolation requires longer names is that instead of crate, it uses _crate, and it adds the module name to the directory - that is duplicated twice, and so any more characters than the existing name blows up.

[igfoo]

There was some @codeqls in the comments at the start of this file; presumably they need to be changed too?

[criemen]

I think I changed the relevant ones - when you build from the internal repo, we have an alias in place that you can still call the build targets @codeql//something, only from the external repo it's now (unfortunately) @ql//something, which is why I changed the wording of the comment there to remove the repo name alltogether when building from the external repo.
That said, there might still be an incorrect comment there, so if you find one, please shout!

[redsun82]

I think we can avoid having to change @codeql to @ql by using repo_name here as well:

Suggested change

	module(
	name = "codeql",
	name = "ql",
	version = "0.0",
	)
	module(
	name = "ql",
	version = "0.0",
	repo_name = "codeql",
	)

[criemen]

This is an excellent and unexpected find, thanks! This indeed makes things much nicer.

[redsun82]

I'm good with these changes, thanks! I'd prefer to keep ruby_deps as an alias of rd though, as you can do that with use_repo (see the kwargs parameter in the docs).

[redsun82]

really minor cosmetic nit, but is there any reason to use cp here and just r for ruby? If cr for ruby is already too much, maybe then also use p here?

[criemen]

Yeah, I wanted to limit the use of single-letter repo names to where we really need them (as there's only 26 of them available). The more letters, the easier to search for the repo name in our source code, too.

[redsun82]

You can also assign an alias here, avoiding the changes in the two build files:

Suggested change

	use_repo(r, "rd")
	use_repo(r, ruby_deps="rd")