Use pwd as a workspace; move sources to src/gopheros and rewrite imports

By setting up pwd as a Go workspace, we can trim import paths from
something like "github.com/achilleasa/gopher-os/kernel" to just
"kernel".

These changes make forking easier and also allows us to move the code to
a different git hosting provider without having to rewrite the imports.

src/gopheros/kernel/kmain/kmain.go

@@ -0,0 +1,45 @@
+package kmain
+
+import (
+	"gopheros/kernel"
+	"gopheros/kernel/goruntime"
+	"gopheros/kernel/hal"
+	"gopheros/kernel/hal/multiboot"
+	"gopheros/kernel/mem/pmm/allocator"
+	"gopheros/kernel/mem/vmm"
+)
+
+var (
+	errKmainReturned = &kernel.Error{Module: "kmain", Message: "Kmain returned"}
+)
+
+// Kmain is the only Go symbol that is visible (exported) from the rt0 initialization
+// code. This function is invoked by the rt0 assembly code after setting up the GDT
+// and setting up a a minimal g0 struct that allows Go code using the 4K stack
+// allocated by the assembly code.
+//
+// The rt0 code passes the address of the multiboot info payload provided by the
+// bootloader as well as the physical addresses for the kernel start/end.
+//
+// Kmain is not expected to return. If it does, the rt0 code will halt the CPU.
+//
+//go:noinline
+func Kmain(multibootInfoPtr, kernelStart, kernelEnd uintptr) {
+	multiboot.SetInfoPtr(multibootInfoPtr)
+
+	hal.InitTerminal()
+	hal.ActiveTerminal.Clear()
+
+	var err *kernel.Error
+	if err = allocator.Init(kernelStart, kernelEnd); err != nil {
+		panic(err)
+	} else if err = vmm.Init(); err != nil {
+		panic(err)
+	} else if err = goruntime.Init(); err != nil {
+		panic(err)
+	}
+
+	// Use kernel.Panic instead of panic to prevent the compiler from
+	// treating kernel.Panic as dead-code and eliminating it.
+	kernel.Panic(errKmainReturned)
+}