install,export: Do not treat language names as target names

When generating `IMPORTED_LINK_INTERFACE_LANGUAGES`, do not treat the
entries as target names.

Fixes: #19846
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

00:18:01

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:45:04

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:01:23

passed cmake-moonshade-osx-debug-ninja

00:11:55

passed cmake-moonshade-osx-release-makefiles

00:11:32

passed cmake-terfin-windows-release-vs2019

00:35:57

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager