Avoid filename collision in the monorepo structure

[thenorili]

Error 1 below appears in both stable and nightly builds.

It's slightly more impactful than just a warning. Running cargo +nightly test, cargo test, cargo +nightly test in windows produces a linker error where color-spantrace tries to use eyre's usage.exe, see error 2 below. It's worked around pretty easily with cargo clean, but it's a problem that might get worse as the monorepo effort expands!

[1]
warning: output filename collision.
The example target usage in package eyre v0.6.8 (C:\Users\nori\dev\eyre\eyre) has the same output filename as the example target usage in package color-spantrace v0.2.0 (C:\Users\nori\dev\eyre\color-spantrace).
Colliding filename is: C:\Users\nori\dev\eyre\target\debug\examples\usage.exe
The targets should have unique names.
Consider changing their names to be unique or compiling them separately.
This may become a hard error in the future; see rust-lang/cargo#6313.

[2]
= note: LINK : fatal error LNK1104: cannot open file 'C:\Users\nori\dev\eyre\target\debug\examples\usage.exe'

error: could not compile color-spantrace (example "usage") due to previous error