Update 21 migration to use existing util for import check

[damyanpetev]

@copilot Yes, that'd be an improvement and even better there's already an namedImportFilter in projects\igniteui-angular\migrations\common\tsUtils.ts that does a similar check for other migrations that should've been used to begin with.
Since this one is merged, create a new PR against master to update this migration to use the tsUtil and while at it adjust the namedImportFilter function to also handle subpaths because the current endWith check handles @infragistics/igniteui-angular as well, but limits the subpaths for entry points after the main package name.

Originally posted by @damyanpetev in #16547 (comment)