Consolidate migration tool into single ps1 file

Merge PSGetMigration.psm1/psd1 module into ConvertTo-PSResourceGet.ps1
as a self-contained script. All functions are defined inline and the
main entry point is guarded so dot-sourcing for tests works correctly.

Removes: PSGetMigration.psm1, PSGetMigration.psd1

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: adityapatwardhan