Cargo has a few odd (old) ways of picking source files if the `bin.path`
attribute isn't given in the Cargo.toml. This commit adds support for
some of those. The previous behaviour always defaulted to `src/main.rs`
which was not always the right choice.

Since there is  look-ahead into the unpacked sources before running the
actual builder the path selection logic has to be embedded within the
build script.

`buildRustCrate` currently supports two ways of running building
binaries when processing a crate:

- Explicit definition of all the binaries (& optionally the paths to
their respective `main.rs`) and,
- if not binary was explictly configured all files matching the patterns
  `src/main.rs`, `src/bin/*.rs`.

When the explicit list is given without path information paths are now
being picked from a list of candidates. The first match wins. The order
is the same as within the cargo compatibility code.

If the crate does not provide any libraries the path `src/{bin_name}.rs`
is also considered.

All underscores within the binary names are translated into dashes (`-`)
before the lookups are made. This seems to be a common convention.

The build expression got quiet large over time and to make it a bit
easier to grasp the different scripts involved in the build are now
separated from the nix file.

This commit adds test based on real-world crates (brotli).
There were a few more edge cases that were missing beforehand. Also it
turned out that we can get rid of the `finalBins` list since that will
now be handled during runtime.

buildRustCrate: add heuristic to picking the right binary source files