Add support for appmap_dir, use correct class loader

[apotterri]

Also, rearrange some code allow an in-memory file system implementation to be used during unit testing.

Fixes #198 . Fixes #186 .

[appland-release]

:tada: This PR is included in version 1.20.0 :tada:

The release is available on:

• v1.20.0
• GitHub release

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