Just in case you need it, here is aptitude output when trying to purge libegl1-mesa:amd64 while libegl1 is already installed
root@blackhole ~ # LANG=C aptitude purge libegl1-mesa:amd64
The following packages will be REMOVED:
libegl1-mesa{p}
0 packages upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 65.5 kB will be freed.
The following packages have unmet dependencies:
weston : Depends: libegl1-mesa (>= 8.0-2) but it is not going to be installed or
libegl1-x11 which is a virtual package and is not provided by any available package
The following actions will resolve these dependencies:
Remove the following packages:
1) weston [6.0.1-2 (now, testing)]
Accept this solution? [Y/n/q/?] n
The following actions will resolve these dependencies:
Keep the following packages at their current version:
1) libegl1-mesa [19.1.6-1 (now, testing)]
Accept this solution? [Y/n/q/?] n
*** No more solutions available ***
The following actions will resolve these dependencies:
Keep the following packages at their current version:
1) libegl1-mesa [19.1.6-1 (now, testing)]
Accept this solution? [Y/n/q/?] q
Abandoning all efforts to resolve these dependencies.
Abort.