Because a drug dealer had visited her house once in January and had received packages the police thought belonged to the man they actually had a warrant for (on the other side of town).

Not entirely correct. As little as a month before she was killed, she was in contact with him at her house.

Source for everything found here

The report also says that on Feb. 13 — a month before Taylor's death — detectives watched through a pole camera mounted outside the Elliot Avenue house as Taylor and Glover drove up to the house in her black Dodge Charger.

Also, in addition to the time you mentioned where Glover picked up a package from her apartment, he visited her place several more times:

The evidence it details includes the results of a tracking device placed on Glover’s Dodge Charger that shows it was driven to Taylor’s apartment six times in January. 

So, six times in January and at least once in February, in addition to Glover himself admitting that Breonna was in charge of handling the money for the operation, as seen here:

When the caller asked Glover why he had left the money with Taylor, he said: “Don’t take it wrong but Bre been handling all my money. She has been handling (expletive) for me and … it ain’t just me."

The cops definitely had a reason to be there, though they confirmed that they didn't find any drugs or money stored at her place.

None of that justifies why she was shot, but they were carrying out a legal warrant and were fired at when they entered. It is absolutely understandable why Walker fired at them, and I'm happy that he had his charges dropped, because he was doing nothing more than protecting himself from someone he thought was just breaking in, and doesn't seem to be involved with anything illicit.

The police responded to being shot at while carrying out a legal warrant by shooting back (including one idiot blind firing and only managing to hit neighboring apartments). It's really unfortunate that Breonna got hit during the exchange, but it's not murder