Update FlattenPdbs Plugin to pick up additional symbols.

[joschock]

Description

Modifies the existing FlattenPdbs plugin to a) pick up additional .PDB files - remove the restriction that they must come from the OUTPUT directory, and b) also include *.debug symbol files.

• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?

How This Was Tested

Executed a build, confirmed that additional symbol files now present in the PDB flat directory.

Integration Instructions

N/A

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/202502@25b93e3). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/202502    #1616   +/-   ##
=================================================
  Coverage                  ?    2.22%           
=================================================
  Files                     ?     1460           
  Lines                     ?   381059           
  Branches                  ?     4584           
=================================================
  Hits                      ?     8477           
  Misses                    ?   372509           
  Partials                  ?       73           

Flag	Coverage Δ
MdeModulePkg	1.55% <ø> (?)
MdePkg	5.46% <ø> (?)
NetworkPkg	0.55% <ø> (?)
PolicyServicePkg	30.42% <ø> (?)
UefiCpuPkg	4.92% <ø> (?)
UnitTestFrameworkPkg	11.40% <ø> (?)

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