sparse: move config parsing into core This patch marks the beginning of moving code from the sparse extension into core. The goal is to move as much of the functionality as possible into core, where it will be an experimental feature. The extension will likely continue to exist to enable the feature and provide UI elements. As part of the move, the repo method was converted to a module function. It doesn't need to exist on repos. An error message was also updated to reflect that an error isn't necessarily from the .hg/sparse file. The API should be updated later to pass in a filename so the error can be more descriptive. Copyright of the added file was copied from the sparse extension.

sparse: use vfs.tryread() vfs.exists() followed by a file read is an anti-pattern because it incurs an extra stat() to test for file presence. vfs.tryread() returns empty string on missing file and avoids the stat().

sparse: refactor sparsechecksum() This was relying on garbage collection to close the opened file, which is a bug. Both callers simply called into self.vfs to resolve the path. So refactor to use the vfs layer. While we're here, rename the method to reflect it is internal and to break anyone relying on the old behavior.