Refactor how extensions are built in compute Dockerfile

1. Rename some of the extension layers, so that names correspond more
   precisely to the upstream repository name and the source directory
   name. For example, instead of "pg-jsonschema-pg-build", spell it
   "pg_jsonschema-build". Some of the layer names had the extra "pg-"
   part, and some didn't; harmonize on not having it. And use an
   underscore if the upstream project name uses an underscore.

2. Each extension now consists of two dockerfile targets:
   [extension]-src and [extension]-build. By convention, the -src
   target downloads the sources and applies any neon-specific patches
   if necessary. The source tarball is downloaded and extracted under
   /ext-src. For example, the 'pgvector' extension creates the
   following files and directory:

    /ext-src/pgvector.tar.gz  # original tarball
    /ext-src/pgvector.patch   # neon-specific patch, copied from patches/ dir
    /ext-src/pgvector-src/    # extracted tarball, with patch applied

    This separation avoids re-downloading the sources every time the
    extension is recompiled. The 'extension-tests' target also uses the
    [extension]-src layers, by copying the /ext-src/ dirs from all
    the extensions together into one image.

This refactoring came about when I was experimenting with different
ways of splitting up the Dockerfile so that each extension would be in
a separate file. That's not part of this PR yet, but this is a good
step in modularizing the extensions.

Author: hlinnaka