Source Locators Are Absolute, but Missing Leading Slash

I noticed that the firtool-produced error messages coming out of scala-cli are not showing up. I think this is due to the source locators being emitted kind of strangely. Consider:

```scala
//> using scala "2.13.10"
//> using lib "org.chipsalliance::chisel::5.0.0-RC1"
//> using plugin "org.chipsalliance:::chisel-plugin::5.0.0-RC1"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
import circt.stage.ChiselStage

class Foo extends Module {
val x = IO(Output(Reset()))
}

object Main extends App {
println(ChiselStage.emitCHIRRTL(new Foo))
println(
ChiselStage.emitSystemVerilog(
gen = new Foo,
firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info")
)
)
}
```

The CHIRRTL is:

```
FIRRTL version 2.0.0
circuit Foo :
module Foo :
input clock : Clock
input reset : UInt<1>
output x : Reset @[Users/schuylere/repos/github.com/llvm/circt/build/scala-cli/Foo.scala 10:13]

skip
```

Note that the source locator is `Users/schuylere/...`. I.e., this is a relative path, but is actually absolute.

The error (top-level module uninferred reset port) is then reported as:

```
Users/schuylere/repos/github.com/llvm/circt/build/scala-cli/Foo.scala:10:13: error: a port "x" with abstract reset type was unable to be inferred by InferResets (is this a top-level port?)
:3:10: note: the module with this uninferred reset port was defined here
module Foo :
^
```

If I manually change the source locator to be absolute, I get:

```
/Users/schuylere/repos/github.com/llvm/circt/build/scala-cli/Foo.scala:10:13: error: a port "x" with abstract reset type was unable to be inferred by InferResets (is this a top-level port?)
val x = IO(Output(Reset()))
^
Foo.fir:3:10: note: the module with this uninferred reset port was defined here
module Foo :
^
```

Reproduce the scala-cli example using ChiselStage.emitCHIRRTL or emitSystemVerilog and inspect how the source locator reaches firtool. Compare the emitted locator with firtool's diagnostic output; done means absolute source paths include their leading slash and diagnostics still point to the source line.

Written by the indexing model from the issue text.