Improve LoadExtension to work correctly with dotnet run and lib* named libs

[krwq]

Currently if you package an SQLite extension in the nuget package by placing binaries in the runtimes/<rid>/native the extension will work fine when you do dotnet run -r <rid> but when using just dotnet run it won't pick up the dll because they end up in different directory than an app. Currently host uses property NATIVE_DLL_SEARCH_DIRECTORIES to provide all paths where native dlls should be searched for but SQLite doesn't know about its existence. In normal scenario of just loading library this will happen automatically but since SQLite doesn't use that logic we need to account for it so this scenario works correctly.

I've also added searching for libraries with lib prefixes because I noticed #24094 - please let me know if we don't want this (SQLite will only make sure correct extension is used for all OSes but it won't take care of the lib prefix).

Might also fix this but I haven't tested android: #24094

[SamMonoRT]

@krwq - changes look straightforward, but I would like a review by @AndriySvyryd and @cincuranet once they are back from vacation to understand the entire workflow.

[cincuranet]

Suggested change

	return searchDirs!.Split([ Path.PathSeparator ], StringSplitOptions.RemoveEmptyEntries);
	return searchDirs.Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries);

[krwq]

I cannot change this because we're building against netstandard 2.0 where I'm getting:

• (for lack of !): error CS8602: Dereference of a possibly null reference.
• (using (char, StringSplitOptions) overload): error CS1503: Argument 2: cannot convert from 'System.StringSplitOptions' to 'char'

[cincuranet]

Some minor code style changes, but otherwise LGTM.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[krwq]

Please hold off with merging until @roji tries out this change on OSX

[roji]

Confirmed that this works on OSX, merging.