[Deferred] Update .groupproj references when a project file is renamed

[darianmiller]

Deferred from #11 (which now renames a program/library/package's
.dpr/.dpk, .dproj, and .res when the module name changes). Updating
.groupproj files was split out because it needs its own file handler
and carries higher risk.

Background

.groupproj is NOT in the default filespec (*.pas;*.dpr;*.dpk;*.dproj)
and, unlike .dproj, has no dedicated handler -- if scanned it would be
mis-tokenized as Pascal. When #11 renames a project's .dproj, any
.groupproj that references the old .dproj filename becomes stale.

A .groupproj references a renamed project in three places:

• <Projects Include="path\Old.dproj">
• <MSBuild Projects="path\Old.dproj"/> (inside the generated targets)
• MSBuild target names derived from the project name by replacing
  non-alphanumeric characters with _ (e.g. Delphi_Lexer_TokenDump,
  plus :Clean / :Make variants and the CallTarget list).

Scope decisions carried over from #11

• Discovery is opt-in: only act on .groupproj when *.groupproj is in
  the effective --filespec (consistent with the .res opt-in in Renaming a program/library/package name should also rename its .dpr/.dproj/.res files #11).
• Target-name rewriting (the dot/dash -> underscore munging) is the
  fragile part and can corrupt the group if imperfect. Decide whether to
  do path references only, or full target-name rewriting, when picking
  this up. Path references (Projects Include / MSBuild Projects) are the
  build-critical ones; target names are cosmetic labels for a normal
  group build.

Fix approach

Add a dedicated raw-text .groupproj handler (mirroring
ProcessDprojFile: operate on the source string to preserve line
endings / BOM). When a project file is renamed via the #11 path, rewrite
matching .dproj path references (and, if in scope, the derived target
names) in every scanned .groupproj.

Acceptance criteria

• .groupproj Projects Include and MSBuild Projects paths updated when a project is renamed
• Target names updated, or explicitly documented as out of scope
• Opt-in via *.groupproj in the filespec; line endings / BOM preserved
• Group builds after a project rename
• Test coverage for a .groupproj reference update