Accept and move to rust2rpm.conf as standard config file location

Using .rust2rpm.conf is confusing to new users, because they never see
these files in existing packages and are only seeing magic happen.

This commit changes the default location of the configuration file to
./rust2rpm.conf, and will rename existing .rust2rpm.conf or _rust2rpm.conf
files to the new name if they already exist.

I have also added a little more documentation about the config file itself
to the README.

README.md

@@ -1,13 +1,33 @@
 # rust2rpm
 
-Convert Rust crates to RPM.
+rust2rpm is a tool for automatically generating RPM spec files for Rust crates.
 
-## `.rust2rpm.conf`
+## `rust2rpm.conf`
 
-You can place configuration file which is used as source for additional
+If it is present, a `rust2rpm.conf` configuration file is read
-information for spec generation.
+by rust2rpm to override some aspects of the automatic spec file generation.
 
-Some simple example would be better than many words ;)
+This file can be committed to dist-git to ensure that these settings
+will be applied for future updates of a crate package, as well.
+
+The file follows a slightly modified `ini` syntax. It supports sections for
+default settings (`[DEFAULT]`) and target-specific settings (i.e. `[fedora]`).
+
+These configuration options are available right now:
+
+- `unwanted-features`: features or optional dependencies for which no
+  `+feature` subpackage should be generated (for example, dependencies on Rust
+  compiler internals for crates that are also bundled with Rust itself)
+- `buildrequires`: additional build dependencies that must be installed for the
+  package to build correctly (i.e., `pkgconfig(foo)` for the `foo-sys` crate)
+- `lib.requires`: additional build dependencies that must be installed for the
+  crate to build correctly as a dependency of another crate (usually, this
+  contains the same values as the `buildrequires` option)
+- `lib+foo.requires`: additional build dependencies that must be installed for
+  the crate to build correctly as a dependency of another crate if it has
+  enabled the optional `foo` feature
+- `bin.requires`: additional runtime dependencies of the binary application that
+  is built from this crate
 
 ```ini
 [DEFAULT]
@@ -27,3 +47,10 @@ lib.requires =
 lib+default.requires =
   pkgconfig(bar) >= 2.0.0
 ```
+
+Note that features and optional dependencies that are marked as
+`unwanted-features` must not be dependencies of other Cargo features that are
+not marked "unwanted" as well. Failing to consider transitive dependencies will
+lead to broken dependencies of generated `rust-foo+bar-devel` packages and / or
+errors when generating spec files with rust2rpm.
+

rust2rpm/__main__.py

@@ -498,7 +498,22 @@ def main():
     kwargs["license_files"] = license_files
 
     conf = configparser.ConfigParser(interpolation=configparser.ExtendedInterpolation())
-    conf.read(["_rust2rpm.conf", ".rust2rpm.conf"])
+    confs = conf.read([".rust2rpm.conf", "_rust2rpm.conf", "rust2rpm.conf"])
+
+    # clean up configuration files with deprecated names
+    if len(confs) > 1:
+        print("WARNING: More than one *rust2rpm.conf file is present in this directory.")
+        print("         Ensure that there is only one, and that it has the correct contents.")
+        sys.exit(1)
+
+    if ".rust2rpm.conf" in confs and "rust2rpm.conf" not in confs:
+        os.rename(".rust2rpm.conf", "rust2rpm.conf")
+        print("Renamed deprecated, hidden .rust2rpm.conf file to rust2rpm.conf.")
+
+    if "_rust2rpm.conf" in confs and "rust2rpm.conf" not in confs:
+        os.rename("_rust2rpm.conf", "rust2rpm.conf")
+        print("Renamed deprecated _rust2rpm.conf file to rust2rpm.conf.")
+
     if args.target not in conf:
         conf.add_section(args.target)