The timeline of the import_shims removal has evolved
over time; notably, we are keeping the import_shims
in Lilac in order to make any lingering old-style
import paths obvious to operators.
Update the ADR to reflect this. Also, add a table
mapping old import paths to new ones.
The old folder name is somewhat confusing, because the
folder contains shims to _compensate for the removal
of sys.path hacks_, but does not contain the sys.path
hacks themselves.
Furthermore, this import_shims/ system could also be
used for other import path changes, such as turning
the locally-installed packages in common/lib/
into regular, importable modules
(e.g. `from common.lib.xmodule import abc` instead of
`from xmodule import abc`). So, a name that is not
specific to the sys.path hacks may be better
in the medium-to-long term.
Along the same lines, we also rename SysPathHackWarning
to DeprecatedEdxPlatformImportWarning.
