rustc build-depends on self?

After reading jxself's comment here,[1] I decided to take a stab at
writing a package helper to rebrand Rust.

I got as far as a find-and-replace from "rust" to "alanguage" in
file/directory names and file contents.[2] (This isn't actually proper.
Some of the replacements are part of things like URLs and will need to
be reverted, but for now I'm just trying to get the package to compile
after rebranding.) I then ran the package helper, tried to compile the
resulting source package with sbuild, and got this error:

```
The following packages have unmet dependencies:
sbuild-build-depends-alanguagec-dummy : Depends: alanguagec (>=1.34.0+dfsg) but it is not installable
Depends: alanguagec (< 1.36) but it is not installable
E: Unable to correct problems, you have held broken packages.
```

Apparently the rustc source package build-depends[3] on rustc binary
package, and the rustc binary package[4] is provided by the rustc source
package. It is possible to build the rustc source package anyway,
because the rustc binary package is already in Trisquel's repository,
but I'm not sure how to build alanguagec when the source package
build-depends on one of its own binaries.

I ran into a similar apparent circular build dependency a little while
back with Qt.[5] I suspect that I have some fundamental misunderstanding
of the relationship between source and binary packages or of how source
packages are built. Can anyone more knowledgeable about Debian packaging
shed some light?

[1] https://trisquel.info/en/forum/rust-non-free#comment-142921

[2] https://devel.trisquel.info/chaosmonk/package-helpers/blob/alanguage/helpers/make-rustc

[3] https://packages.ubuntu.com/source/xenial/rustc

[4] https://packages.ubuntu.com/xenial/rustc

[5] https://trisquel.info/en/forum/apparent-circular-build-dependencies