Fix absolute path handling in SemanticdbTaskListener.absolutePathFromUri

[lihaoyi]

The previous code was off by one:

• If given a relative path vf://tmp/foo/bar/baz, it would crash with unsupported URI
• If given an absolute path vf://tmp//foo/bar/baz, it would incorrectly produce a relative path foo/bar/baz using the absolute path's segments, which would typically duplicate the -sourceroot value and result in an invalid path

Looking at the code, it seems like it is simply splitting by 1 too many slashes: if we want everything after the vf://tmp/, we need to .split('/', 4) rather than .split('/', 5). This seems fixes both misbehaviors listed above:

• If given a relative path vf://tmp/foo/bar/baz, it extracts the relative foo/bar/baz, which is then appended as a sub-path by options.sourceroot.resolve
• If given an absolute path vf://tmp//foo/bar/baz, it extracts the absolute /foo/bar/baz, which then remains unchanged and is returned by options.sourceroot.resolve

Test plan

Tested manually, a local sbt publishLocal with these changes fixes downstream issues in tools that consume the semanticdb e.g. scalameta/metals#7913

[lihaoyi]

CC @antonsviridov-src since you seem to be the most active maintainer