[escher] Clean up shader file initialization.

HackFilesystem was becoming messy.  The original idea was to encapsulate
platform-specific functionality in subclasses, but eventually we needed
to conditionally add Fuchsia-specific functionality to the interface.

It became simpler to delete the subclasses and just use #ifdef in the
single implementation, so that's what I did.

Test: manual session add escher_rainfall_on_scenic.cm/escher-flatland.cm
Test: manual paper_shader_compiler
Bug: fxb/494718197
Change-Id: I0b245733702b0b8fe9a593ae4137220e60993e72
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1550982
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>
Reviewed-by: Caroline Liu <carolineliu@google.com>
Commit-Queue: Josh Gargus <jjosh@google.com>