Move main.go from fxconfig/cmd to fxconfig/ (#68)

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#### Type of change

<!--- What type of change? Pick one or more options and delete the
others. -->

- Improvement

#### Description

This fix allows to install `fxconfig` using the command:

```shell
go install github.com/hyperledger/fabric-x/tools/fxconfig@latest
```

The current command `go install
github.com/hyperledger/fabric-x/tools/fxconfig/cmd@v0.0.10` produces a
binary named `cmd`, which is not great when people install `fxconfig` on
their systems.

Signed-off-by: pco <pasquale.convertini@ibm.com>

Author: pasquale95

Dockerfile

@@ -27,7 +27,7 @@ COPY . .
 RUN go build -o /tmp/bin/configtxgen ./tools/configtxgen
 RUN go build -o /tmp/bin/cryptogen ./tools/cryptogen
 RUN go build -o /tmp/bin/configtxlator ./tools/configtxlator
-RUN go build -o /tmp/bin/fxconfig ./tools/fxconfig/cmd
+RUN go build -o /tmp/bin/fxconfig ./tools/fxconfig
 
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 # Stage 2: Production runtime image