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/mem/vmm/page.go

@@ -0,0 +1,19 @@
+package vmm
+
+import "gopheros/kernel/mem"
+
+// Page describes a virtual memory page index.
+type Page uintptr
+
+// Address returns a pointer to the virtual memory address pointed to by this Page.
+func (f Page) Address() uintptr {
+	return uintptr(f << mem.PageShift)
+}
+
+// PageFromAddress returns a Page that corresponds to the given virtual
+// address. This function can handle both page-aligned and not aligned virtual
+// addresses. in the latter case, the input address will be rounded down to the
+// page that contains it.
+func PageFromAddress(virtAddr uintptr) Page {
+	return Page((virtAddr & ^(uintptr(mem.PageSize - 1))) >> mem.PageShift)
+}