How to: Target a local android dependency

I'm trying to modify a flutter package but the part that needs changing is in an Android dependency being used by that package.

The flutter package is epub_viewer (https://pub.dev/packages/epub_viewer) which implements a modified version of FolioReader (https://github.com/JideGuru/Folioreader-Android) ...

I've been trying to follow examples on here but it doesn't make sense and keeps telling me it can't find the package https://developer.android.com/studio/build/dependencies

My file structure is like this:

../users/documents/epub_viewer
../users/documents/folioreader

and I need to change this dependency in epub_viewer's app build.gradle

dependencies {
    implementation "com.jideguru:folioreader:0.6.2"
    ...
}