Prevent adding '-' CLI flag to allowed paths as if it was an actual file

- This resulted in a bug where using stdin was effectively disabling allowed path checks.

test/cmdlineTests/stdin_allowed_paths/args

@@ -0,0 +1 @@
+-

test/cmdlineTests/stdin_allowed_paths/err

@@ -0,0 +1,11 @@
+Error: Source "too_long_line/input.sol" not found: File outside of allowed directories.
+ --> <stdin>:4:1:
+  |
+4 | import "../too_long_line/input.sol";
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Error: Source "error_codes/input.sol" not found: File outside of allowed directories.
+ --> stdin_allowed_paths/input.sol:4:1:
+  |
+4 | import "../error_codes/input.sol";
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

test/cmdlineTests/stdin_allowed_paths/exit

@@ -0,0 +1 @@
+1

test/cmdlineTests/stdin_allowed_paths/input.sol

@@ -0,0 +1,4 @@
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity >=0.0;
+
+import "../error_codes/input.sol";

test/cmdlineTests/stdin_allowed_paths/stdin

@@ -0,0 +1,4 @@
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity >=0.0;
+
+import "../too_long_line/input.sol";