Give build/run/check/test/doc a -file flag.

A package has canonically always been a directory, but odin allowing you to build a single-file package confused newcomers who didn't understand why they could then not access variables and procedures from another file in the same directory.

This change disallows building single-file packages by default, requiring the `-file` flag to acknowledge you understand the nuance.

`-help` for these commands also clarifies the difference.

```
W:\Odin>odin build -help
odin is a tool for managing Odin source code
Usage:
        odin build [arguments]

        build   Compile directory of .odin files as an executable.
                One must contain the program's entry point, all must be in the same package.
                Use `-file` to build a single file instead.
                Examples:
                        odin build .                    # Build package in current directory
                        odin build <dir>                # Build package in <dir>
                        odin build filename.odin -file  # Build single-file package, must contain entry point.

        Flags

        -file
                Tells `odin build` to treat the given file as a self-contained package.
                This means that `<dir>/a.odin` won't have access to `<dir>/b.odin`'s contents.
```

```
W:\Odin>odin run examples\demo\demo.odin
ERROR: `odin run` takes a package as its first argument.
Did you mean `odin run examples\demo\demo.odin -file`?
The `-file` flag tells it to treat a file as a self-contained package.
```

src/build_settings.cpp

@@ -598,7 +598,6 @@ bool allow_check_foreign_filepath(void) {
 	return true;
 }
 
-
 // TODO(bill): OS dependent versions for the BuildContext
 // join_path
 // is_dir
@@ -606,6 +605,28 @@ bool allow_check_foreign_filepath(void) {
 // is_abs_path
 // has_subdir
 
+enum TargetFileValidity : u8 {
+	TargetFileValidity_Invalid,
+
+	TargetFileValidity_Writable_File,
+	TargetFileValidity_No_Write_Permission,
+	TargetFileValidity_Directory,
+
+	TargetTargetFileValidity_COUNT,
+};
+
+TargetFileValidity set_output_filename(void) {
+	// Assembles the output filename from build_context information.
+	// Returns `true`  if it doesn't exist or is a file.
+	// Returns `false` if a directory or write-protected file.
+
+
+
+
+	return TargetFileValidity_Writable_File;
+}
+
+
 String const WIN32_SEPARATOR_STRING = {cast(u8 *)"\\", 1};
 String const NIX_SEPARATOR_STRING   = {cast(u8 *)"/",  1};