Add the ability to specify a custom source-directory

[astrofrog]

This is useful for e.g. monorepos or in general repositories where the pyproject.toml may not be at the root of the repository

[astrofrog]

I thought maybe build-directory makes more sense so I've changed it to that, but open to suggestions.

[ConorMacBride]

Good idea! I see the option is called srcdir in the build docs, so maybe source-directory would be fine? build-directory sounds like it could refer to a build/ directory.

(Don't think it should be called working-directory in this repo as it doesn't affect where build artifacts are written to.)

[astrofrog]

@ConorMacBride - does this seem ok?

[ConorMacBride]

Looks good!