Check for case insensitive License.txt when RequireLicense is specified

[alerickson]

PR Summary

Currently the check for License.txt is case sensitive, so other variations, including license.txt are not found. This results in an error message specifying that there is no License.txt file, when in fact there is. This PR loops through all files at the root of a module to search for a case insensitive License.txt.

This change can only be tested interactively due to prompting.

PR Context

Resolves #1729

PR Checklist

• [x] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [x] Summarized changes
• [x] Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• [x] This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • [x] None
  • OR
  • [ ] [Documentation needed]()
    • [ ] Issue filed:
• User-facing changes
  • [x] Not Applicable
  • OR
  • [ ] [Documentation needed]()
    • [ ] Issue filed:
• Testing - New and feature
  • [x] N/A or can only be tested interactively
  • OR
  • [ ] Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • [x] I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • [ ] I have considered the user experience from a tooling perspective and enumerated concerns in the summary.