Rename most av1/aom symbols and files to av2/avm

As of b52eef5c, https://gitlab.com/AOMediaCodec/avm cannot be used in the same compilation unit as https://aomedia.googlesource.com/aom, making integration into tools and libraries difficult.
Most file, directory, function, variable and library names were forked from aom to avm. This generates a lot of conflicts when the two code bases are compiled as dependencies of another project. For example: ld: error: duplicate symbol: av1_get_MBs.

See https://github.com/AOMediaCodec/libavif/pull/1361#discussion_r1183063857.