putting slash in object name causes build failure

[goshhhy]

if using std.build.Builder.addObject in build.zig with an object name that has a slash in it, the build fails due to "no such file or directory" as it attempts to store the resulting object in a directory inside zig-cache/o that does not exist.

this behavior was not something i expected. for the use case of converting existing (large) projects in c to use the zig build system, and integrating zig files into it, being able to have the object name and path be the same string greatly simplifies writing the build.zig file.

perhaps the build system can check for this case, and automatically create the subdirectory path for the object file?

[andrewrk]

Related: #2279

At the very least zig should reject an object name with slashes in it.