The existing reference shortener does not use import alias when it
shortens a symbol. Instead, it adds a new import directive for the
symbol that is already imported. This commit updates reference shortener
to let it reuse the existing import alias rather than adding a new one:
1. When shortening a symbol, check whether the symbol is already
imported.
2. If it is already imported by an import alias, keep the symbol
reference expression and the import alias as a string together in
`ShortenCommand`.
The actual PSI update (shortening) based on the ShortenCommand is done
by IntelliJ.
^KTIJ-27205
Jaebaek Seo
e80f044847