Fix: Resolve main_file path fully during __init__.

This helps avoid problems with relative dir issues. For example,
running:

```
tartex -b Path/To/main.tex
```

led to a crash where the bib file path was being resolved incorrectly as
`/whatever/Path/To/Path/To/main.bib` because the input_files() function
is called after a cd to `Path/To` directory already. Resolving the
main_file path fully at initialisation avoids such rel path issues going
forward.

src/tartex/tartex.py

@@ -212,7 +212,7 @@ class TarTeX:
     def __init__(self, args):
         self.cwd = Path(os.getcwd())
         self.args = parse_args(args)
-        self.main_file = Path(self.args.fname)
+        self.main_file = Path(self.args.fname).resolve()
         if self.main_file.suffix not in [".fls", ".tex"]:
             sys.exit("Error: Source filename must be .tex or .fls\nQuitting")