src/plugins/ctf/common: restructure subtree

src/plugins/ctf/common: restructure subtree

This patch restructures the `src/plugins/ctf/common` subtree as such:

`src`:
    Everything common to source component classes.

`src/metadata/tsdl`:
    TSDL-specific code.

    As of this patch, everything which was part of
    `src/plugins/ctf/common/metadata` is moved here, but some code will
    be shared with CTF 2 sources eventually and move back to
    `src/plugins/ctf/common/src/metadata`.

`print.hpp` isn't needed anymore.

The dummy `ctf-common-src.cpp` exists to make sure the
`libbabeltrace2-plugin-ctf-common-src.la` convenience library has at
least one source file.

This new structure will make it easier to add common `ctf` plugin code
for both source and sink component classes, as well as to add code
specific to CTF 2 and common to both CTF 1 and CTF 2.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4246d6ba37ff26efa5d4b7b4be2ec8600d3207e4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7839