Add safe guards for relative paths

This commit adds multiple safe guards for relative paths, ensuring they
never traverse outside the working directory.

The `SafeRelativePath` flag calculates the safe relative path based on a
relative base dir, which results in a flattened path.

The write methods of `manifestgen` make use of the `SecureJoin` as well,
to ensure writes are never outside of the given directory when used as
a lib outside of the CLI.

Signed-off-by: Hidde Beydals <hello@hidde.co>

go.sum

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@@ -33,24 +32,20 @@ github.com/Azure/go-autorest v14.2.0+incompatible h1:V5VMDjClD3GiElqLWO7mz2MxNAK
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@@ -60,7 +55,6 @@ github.com/Azure/go-autorest/autorest/validation v0.1.0/go.mod h1:Ha3z/SqBeaalWQ
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@@ -142,11 +136,12 @@ github.com/cpuguy83/go-md2man/v2 v2.0.0 h1:EoUDS0afbrsXAZ9YQ9jdu/mZ2sXgT1/2yyNng
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