rust: internalize srcPathFromModuleSrcs

This was frequently misused (for example, in the prebuilts module, it
was used as a complex "assert(len(srcs))==1"), and can be superceded by
getCrateRoot anywhere it was used. It's now only called from
compiler.go, and can drop the second return parameter, as it was only
actually used by the prebuilt assert misuse.

Bug: 309943184
Test: m nothing
Change-Id: I6c92580bc8f0ecb7586c544056b5409e6dd280e7