[repo-assist] Fix find-packages failing on ~ home-directory local NuGet feed paths

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Closes #4243

Root cause

Dependencies.FindPackagesByNameAsync in src/Paket.Core/PublicAPI.fs (used by the paket find-packages CLI command) handled the LocalNuGet(s,_) case by passing the raw source path s directly to Fake.Globbing.search, without normalizing it first.

Other local-feed code paths (NuGet.fs, NuGetLocal.fs) already call Utils.normalizeLocalPath before resolving a local feed directory, which expands a ~/ home-directory shorthand and resolves relative paths correctly. FindPackagesByNameAsync was missing this step, so a source like ~/nuget was searched literally as <cwd>/~/nuget, producing the reported DirectoryNotFoundException.

Fix

Added a call to Utils.normalizeLocalPath on the local source path in FindPackagesByNameAsync, extracting the resolved string from the returned AbsolutePath/RelativePath case before passing it to Fake.Globbing.search. This matches the pattern already used elsewhere in the codebase.

Trade-offs

• Minimal, surgical change limited to the local-NuGet-feed branch of FindPackagesByNameAsync.
• Genuinely rooted paths (including UNC paths, which Path.IsPathRooted already treats as rooted) are unaffected, since normalizeLocalPath returns them unchanged as an absolute path.

Test Status

• dotnet build src/Paket.Core/Paket.Core.fsproj -c Debug -f netstandard2.0 — succeeded, no new warnings/errors.
• Added a new regression test FindPackagesByName expands home directory (~) for local NuGet feed in tests/Paket.Tests/UtilsSpecs.fs, which creates a real temp folder under the user's home directory containing a minimal valid .nupkg, and verifies Dependencies.FindPackagesByName locates it via a ~/... source. Ran with dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~FindPackagesByName" -f net10.0 — Passed (1/1).
• Did not run the full UtilsSpecs suite in this PR; a prior run of the broader filter showed 4 pre-existing, unrelated failures in http_proxy/https_proxy environment-variable tests that are not affected by this change.

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