[repo-assist] Fix local NuGet source not recognizing .symbols.nupkg packages

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Closes #1587

Root cause

When a local NuGet folder source (source c:/... in paket.dependencies) contains only a symbols package (e.g. PackageName.1.2.3.symbols.nupkg), Paket failed to find/version it:

• getAllVersionsFromLocalPath in NuGetLocal.fs explicitly filtered out any file ending in .symbols.nupkg before matching versions.
• The shared parsePackageInfoFromFileName helper in NuGetCache.fs (used by findLocalPackage/tryFindLocalPackage as a fallback lookup) also failed to parse .symbols.nupkg filenames, since its regex expects a bare .nupkg suffix.

Fix

Both functions now strip a trailing .symbols from the filename before matching/parsing package name and version, so a symbols-only package in a local feed resolves identically to its normal counterpart.

Trade-offs

• Minimal, surgical change scoped to filename parsing; no behavior change for non-symbol packages.
• Does not change how a *.nupkg and its .symbols.nupkg sibling are disambiguated when both exist (existing filter/precedence for the main .nupkg still applies).

Test Status

• Added test cases to NuGetLocalSpecs.fs covering .symbols.nupkg filenames for parsePackageInfoFromFileName.
• dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~NuGetLocalSpecs": net9 target passed (2/2). The net461 target could not execute in this sandbox because mono is not installed (pre-existing infrastructure limitation, unrelated to this change).

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