Compression packs symlinks as copies

**Description:**

`sam package` compresses symlinks as an additional copy of the file to which they resolve. Given a traditional shared library layout with

```
libz.so → libz.so.1
libz.so.1 → libz.so.1.2.11
libz.so.1.2.11
```

… `sam package` will pack the library three times, not once with two symlinks.

**Steps to reproduce the issue:**

`make` in a directory with `.env`, `template.yml`, and `Makefile`:

`.env`: replace the values with your own profile, region, and S3 bucket name:
```
AWS_PROFILE=default
AWS_REGION=ap-southeast-2
BUCKET=your-bucket-name-I-do-not-want-to-know
```

`template.yml`:
```yaml
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: Reproducing a SAM CLI compression wart

Globals:
Function:
Runtime: nodejs8.10

Resources:
DummyFunction:
Type: AWS::Serverless::Function
Properties:
FunctionName: Dummy
CodeUri: ./build
Handler: lambda.dummy
```

`Makefile`:
```make
# Import AWS_PROFILE, AWS_REGION, BUCKET, and S3_PREFIX from ./.env:
-include .env

# Set defaults, with none of which you'll likely be happy:
AWS_PROFILE ?= default
AWS_REGION ?= ap-southeast-2
BUCKET ?= your-bucket-name-here
S3_PREFIX ?= lambdas

# Targets
EXPECTED := expected.zip
OBSERVED := observed.zip

# Satisfy `aws` and `sam`:
export AWS_PROFILE
export AWS_REGION

# These targets don't produce files:
.PHONY: comparison clean

# This target is our default:
comparison: $(EXPECTED) $(OBSERVED)

# This cleans up:
clean:
rm -rf build $(OBSERVED) $(EXPECTED)

# Construct the ZIP file we wish SAM had made:
$(EXPECTED): build
( cd build ; zip -9y ../$(EXPECTED) * )
unzip -l $(EXPECTED)

# Download and list the contents of the ZIP file `sam package` uploaded:
$(OBSERVED): packaged.yml
aws s3 cp `grep CodeUri packaged.yml | awk '{ print $$2 }'` $(OBSERVED)
unzip -l $(OBSERVED)

# Zip the `build` directory, upload it to `BUCKET`, and prepare `packaged.yml`:
packaged.yml: build template.yml
sam package \
--debug \
--template-file template.yml \
--s3-bucket $(BUCKET) \
--s3-prefix $(S3_PREFIX) \
--output-template-file packaged.yml

# Prepare a `build` directory with a random 10MB shared library with
# traditional symlinks:
build: Makefile
rm -rf build
mkdir -p build
echo 'module.exports.dummy = function() { return { pass: true }; };' > build/lambda.js
dd if=/dev/random of=build/libz.so.1.2.11 bs=65536 count=160
ln -s libz.so.1.2.11 build/libz.so.1
ln -s libz.so.1 build/libz.so
```
**Observed result:**

```
Archive: observed.zip
Length Date Time Name
--------- ---------- ----- ----
62 06-13-2018 14:02 lambda.js
10485760 06-13-2018 14:02 libz.so.1
10485760 06-13-2018 14:02 libz.so
10485760 06-13-2018 14:02 libz.so.1.2.11
--------- -------
31457342 4 files
```

**Expected result:**

```
Archive: expected.zip
Length Date Time Name
--------- ---------- ----- ----
62 06-13-2018 14:02 lambda.js
9 06-13-2018 14:02 libz.so
14 06-13-2018 14:02 libz.so.1
10485760 06-13-2018 14:02 libz.so.1.2.11
--------- -------
10485845 4 files
```

Also, I expect compression at least as good as `zip -9`.

**Additional environment details (Ex: Windows, Mac, Amazon Linux etc)** macOS 10.13.5

**Output of `sam --version`**: SAM CLI, version 0.3.0

**Optional Debug logs**: I gave `--debug` but couldn't find the output