Ah ok - the reason for the confusion on this point is due to this documentation page.
Where it says:
- A package directory is a directory containing the source trees of a set of packages as subdirectories, and forms an implicit environment. If
Xis a subdirectory of a package directory andX/src/X.jlexists, then the packageXis available in the package directory environment andX/src/X.jlis the source file by which it is loaded.