mpx build will not write files if invoked with `--input src`

[digitalronin]

I've been playing around with mpx ("@mathpix/mpx-cli": "^0.4.0") to build a github pages site from some markdown files with inline and block equations, and noticed some strange behaviour.

Given src/index.mmd, and an empty docs directory, invoking mpx with npx build --input src --output docs results in:

bash-5.1# npx mpx build --input src --output docs
Converting src to docs
Wrote 0 files in 0.24 seconds

If I rename src to source and repeat, it works fine:

bash-5.1# npx mpx build --input source --output docs
Converting source to docs
Wrote 1 file in 0.57 seconds